Just tried the free app "Whistle" which is a SIP client, I tried a bunch of others before finding this one. It works pretty darn nicely. It assigns you a number you get to keep, has caller ID, and the only caveat is when you make a call you hear a real super short ad before it connects you. I don't think there is a minute limitation like on other sip clients, but I really haven't played with it too much yet. Also they don't have bluetooth support yet but are working on it, but it works just fine over the speaker, and also works with a wired headset. Best part about it is if you are jailbroken you can background it, and when you receive a phone call your ipad will ring!! Then you just switch to your whistle app and answer the call. This is all over wifi, haven't tested it on 3g yet, but great stuff.