They're pretty similar. Both have a home screen with icons for programs, a button to return to it, and a similar touch-screen web-browsing interface.
Notable interface differences are the slide-out keyboard on the G1, two-touch zoom gesture on the iPhone, the hardware "back" button on the G1, and the "system-tray" at the top of the G1 that notifies you when a background process has something to say, like you just got a chat or an email or a download finished.
