Line2 was recently reviewed in the NY Times. I was able to get a copy despite some problems, and am very pleased with it so far. Line2 and the associated Toktumi service combine services like Google Voice and Skype. The Toktumi website allows you to setup a new phone number with some fairly advanced call handling options, including different forwarding rules for different callers, auto-attendant, and voicemail. The Line2 app runs on the iPhone. It adds VoIP calling to the mix, allowing calls to be made anywhere you have WiFi access. Voice quality is great, a bit better than SkypeOut calls from my experience.
It can make VoIP calls over 3G, but they recommend against it. Besides, when the service detects that your Line2 app isn't running, incoming calls are instead forwarded to your "real" cell phone number. (you do use your cell phone minutes when in this mode) To make an outgoing call, start the app - if you are connected via WiFi, the outgoing call will use VoIP. If not, it connects to a Toktumi bridge number and connects your call. (this way, the caller ID always shows your Toktumi number)