So apparently many 3rd party accessories don't work with iPhone 3G at the moment. Post the name of your 3rd party accessories and whether they do or don't work in 4 ways: Syncing, charging audio playback via 30 pin connector, and an Incompatibility Message . I'll try to maintain a list.
Name-------------------------------------------Sync---------------------Charge---------------------Audio------Message
Alpine iDA-X001 iPod / media receiver--------[N/A]--------------------[Yes]------------------------[Yes]---------[ ? ]
Alpine x100--------------------------------------[N/A]--------------------[Yes]------------------------[Yes]---------[ ? ]
Altec Lansing IM600-----------------------------[N/A]--------------------[No]-------------------------[Yes]--------[ ? ]
Apple FireWire Power Adapter for iPod--------[N/A]--------------------[No]-------------------------[N/A]--------[ ? ]
Apple iPod Radio Remote-----------------------[N/A]--------------------[N/A]------------------------[No]---------[Yes]
Apple Universal dock---------------------------[Yes]---------------------[Yes]------------------------[ ? ]---------[ ? ]
Belkin iPod dock--------------------------------[Yes]---------------------[Yes]------------------------[ ? ]---------[Yes]
Griffin PowerJolt (w/ retractable USB)--------[N/A]--------------------[Yes]------------------------[N/A]--------[Yes]
Belkin Tunesync--------------------------------[Yes]---------------------[Yes]------------------------[No]---------[No]
Blitzsafe Car/iPod interface--------------------[N/A]---------------------[No]------------------------[Yes]---------[ ? ]
Bose SoundDock--------------------------------[N/A]---------------------[No]------------------------[Yes]---------[ ? ]
Brandmotion iPod adapter for Ford Vehicles--[N/A]---------------------[No]------------------------[Yes]---------[Yes]
Griffin iTrip Autopilot---------------------------[N/A]---------------------[Yes]-----------------------[Yes]---------[No]
Griffin PowerJolt (w/ retractable USB)--------[N/A]--------------------[Yes]------------------------[N/A]--------[Yes]
Griffin TuneJuice Power------------------------[N/A]---------------------[No]------------------------[N/A]---------[Yes]
Harmond Drive+Play---------------------------[N/A]---------------------[No]------------------------[Yes]---------[ ? ]
iHome iH14D table radio/dock-----------------[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
iHome iH4B Alarm Clock-----------------------[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
iHome iH6---------------------------------------[N/A]--------------------[No]-------------------------[No]----------[Yes]
iHome iH8---------------------------------------[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
iLuv i730 car kit (Bluetooth, FM, Charge)-----[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
JBL OnStage II----------------------------------[N/A]---------------------[No]------------------------[Yes]---------[Yes]
Kenwood DDX812 Head Unit w/ iPod cable---[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
MacAlly FMcup----------------------------------[N/A]---------------------[Yes]-----------------------[Yes]----------[ ? ]
Monster iCarPlay Wireless 200 FM-------------[N/A]--------------------[No]-------------------------[Yes]---------[Yes]
Monster iCarPlay Wireless (Plus)--------------[N/A]--------------------[No]-------------------------[Yes]---------[Yes]
Onkyo iPod Dock--------------------------------[N/A]--------------------[Yes]------------------------[Yes]---------[ ? ]
Pioneer FH-P8000BT car stereo----------------[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
Sony ICF-C1iP Dock with Clock----------------[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
USA SPEC Honda iPod Interface---------------[N/A]--------------------[No]-------------------------[Yes]---------[Yes]
XtremeMac | LUNA-----------------------------[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
Name-------------------------------------------Sync---------------------Charge---------------------Audio------Message
Alpine iDA-X001 iPod / media receiver--------[N/A]--------------------[Yes]------------------------[Yes]---------[ ? ]
Alpine x100--------------------------------------[N/A]--------------------[Yes]------------------------[Yes]---------[ ? ]
Altec Lansing IM600-----------------------------[N/A]--------------------[No]-------------------------[Yes]--------[ ? ]
Apple FireWire Power Adapter for iPod--------[N/A]--------------------[No]-------------------------[N/A]--------[ ? ]
Apple iPod Radio Remote-----------------------[N/A]--------------------[N/A]------------------------[No]---------[Yes]
Apple Universal dock---------------------------[Yes]---------------------[Yes]------------------------[ ? ]---------[ ? ]
Belkin iPod dock--------------------------------[Yes]---------------------[Yes]------------------------[ ? ]---------[Yes]
Griffin PowerJolt (w/ retractable USB)--------[N/A]--------------------[Yes]------------------------[N/A]--------[Yes]
Belkin Tunesync--------------------------------[Yes]---------------------[Yes]------------------------[No]---------[No]
Blitzsafe Car/iPod interface--------------------[N/A]---------------------[No]------------------------[Yes]---------[ ? ]
Bose SoundDock--------------------------------[N/A]---------------------[No]------------------------[Yes]---------[ ? ]
Brandmotion iPod adapter for Ford Vehicles--[N/A]---------------------[No]------------------------[Yes]---------[Yes]
Griffin iTrip Autopilot---------------------------[N/A]---------------------[Yes]-----------------------[Yes]---------[No]
Griffin PowerJolt (w/ retractable USB)--------[N/A]--------------------[Yes]------------------------[N/A]--------[Yes]
Griffin TuneJuice Power------------------------[N/A]---------------------[No]------------------------[N/A]---------[Yes]
Harmond Drive+Play---------------------------[N/A]---------------------[No]------------------------[Yes]---------[ ? ]
iHome iH14D table radio/dock-----------------[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
iHome iH4B Alarm Clock-----------------------[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
iHome iH6---------------------------------------[N/A]--------------------[No]-------------------------[No]----------[Yes]
iHome iH8---------------------------------------[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
iLuv i730 car kit (Bluetooth, FM, Charge)-----[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
JBL OnStage II----------------------------------[N/A]---------------------[No]------------------------[Yes]---------[Yes]
Kenwood DDX812 Head Unit w/ iPod cable---[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
MacAlly FMcup----------------------------------[N/A]---------------------[Yes]-----------------------[Yes]----------[ ? ]
Monster iCarPlay Wireless 200 FM-------------[N/A]--------------------[No]-------------------------[Yes]---------[Yes]
Monster iCarPlay Wireless (Plus)--------------[N/A]--------------------[No]-------------------------[Yes]---------[Yes]
Onkyo iPod Dock--------------------------------[N/A]--------------------[Yes]------------------------[Yes]---------[ ? ]
Pioneer FH-P8000BT car stereo----------------[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
Sony ICF-C1iP Dock with Clock----------------[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]
USA SPEC Honda iPod Interface---------------[N/A]--------------------[No]-------------------------[Yes]---------[Yes]
XtremeMac | LUNA-----------------------------[N/A]--------------------[Yes]------------------------[Yes]---------[Yes]