I use a program called Nimbuzz (free IM client) which has support for Gizmo accounts. They recently updated to support multitasking for iOS4. Calling out works perfectly via. Google Voice; my Nimbuzz alerts me after I put in the number on an external GV client, and I can chat through VoIP. Only thing I'm not sure about is regarding incoming calls though (if someone calls my google voice how will the phone figure out whether to answer via VoIP or through the default phone interface?) Ipod touch users who have multitasking can rejoice as well; now with wi-fi you can also VoIP without the need for jailbreaking!