I set Whisle Phone up today and I have now found the answer to a question I wasn't asking....
My only issue with this solution was that I needed to receive calls from land lines. Also, I didn't want to listen to ads before calling. That's why I made mine work using a string of internet providers (sipsorcery.com, google voice, gizmo5, and sipgate.com).
I'm interviewing maker of weephone, one of the apps I used in my free 3G/WiFi calling proof of concept. I hope to see what he things the future of SIP/VoIP calling is for the iPad.