I bought a used 3G in Dec '09, and it's been working fine...even JB. I do NOT have the phone activated. It works like the iPod Touch, but with the camera and vibration mode that the iTouch doesn't have. About 2 months ago, I had to restore the iPhone (no big deal), but I got a message I'd never seen before that said there was "no SIM card", the phone could not be activated (I don't WANT it activated!). Borrowed a SIM from a friend, inserted to my iPhone and it "activated" and I was back in business!! Returned SIM to friend, no problems... NOW, I upgraded to iOS 4 to take advantage of the >180 apps limit and a few other goodies. NOW, I still get a message that there's no SIM installed and I can't do ANYTHING until I install a SIM. Can't keep borrowing a SIM card. The solution, from reading all the Google searches, was to "hacktivate" (JB) it, which I did (yes, there IS a JB for iOS4!). That worked fine for 3-4 days, then the phone went into a constant reboot cycle (the Apple startup screen kept appearing). I've tried RESTORE, it will NOT let me restore to an older OS (3.1.3). Yes, I went into DFU mode, held the OPTION key when clicking the RESTORE button so I could choose the older .ipsw that was saved on my Mac. I get an error that the phone could not be restored (code 1015). If I try to restore regularly, it does with the "latest firmware", but when I then apply the JB (to get around the NO SIM issue) is when it goes into the reboot loop. Bottom line is, I can't use the iPhone at all at this point, not without a SIM (or so it seems). I've been thru Apple's site and tried everything down their trouble-shooting list. NO luck!! AT&T has a CHEAP refurb GoPhone for a whole $7 including shipping if I activate it online (activation fee waived). I don't need or want a voice phone, just a way to use my iPhone the way I've BEEN using it, without service activation. I need a cheap solution to my problem. I've seen some say YES, some say NO. CAN a GoPhone SIM be used to "activate" the iPhone 3G with iOS 4?? Thanks!!