Just read about the ITSP Sipgate and their new app for the Apple iPhone here.  It's a great idea that you'll be able to use a SIP service in the iPhone but it's only going to work using WiFi and not the other Cellular services such as 3G.  Ok, I'm glad that Apple will not 'break' the app and you don't have to 'unlock' the iPhone to make it work but you can't help but hope that one day Apple will 'see the light' and be more open about their products and let SIP run over all networks.  Until then, here's a pic, oh and you'll need an invite from Sipgate to use this app at the moment :)