According to an article I read:
"ICall Inc.'s planned iPhone application is still in beta testing. Like Truphone's app, it will use wifi links to access the VoIP network. A unique feature is the app's ability to switch from a cellular call to a wifi call without interrupting the conversation. ICall claims that such transfers will happen "seamlessly." CEO Arlo Gilbert said that the application should be available in early August 2008."
I should also note that they wont charge for calls like Truphone's
Here's their website: