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.
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Loading Gingerbread through OpeniBoot means the user can still open iOS as well as Android, plus Linux. [@nickpack via CoveringWeb - Thanks, Ahmed!]
DISCUSSION
Every time I see Android on an iPhone, this is what I think of. (censoring was my doing.)