From the same OpeniBoot software that saw other variations of Android running on the iPhone, comes Android 2.3 (Gingerbread), shown on the iPhone 3G. The video is a bit jerky, but it's proof that Gingerbread can be dual-booted.

Loading Gingerbread through OpeniBoot means the user can still open iOS as well as Android, plus Linux. [@nickpack via CoveringWeb - Thanks, Ahmed!]