Probably never. The iPhone lacks the 4 button configuration that's standard on Android phones. It can be made to work, but it will always be a hack.
Also, not to sound like a fanboy, but why would you want Android on your iPhone? I can understand iOS fans' concerns about the UI on Android not being as polished as iOSes, but it seems like if you're going to use Android, using an Android phone just makes sense.
WRT Android on the iPhone, it's like Linux on a Mac: the hardware is usually higher quality. A 3GS and a Samsung Intercept go for the same price on a 2 year contract, but the Intercept feels like a cheap toy compared to the 3GS.
I run Ubuntu on my MacBook as well, but the camera fails sometimes, the brightness control is wonky, the sleep is hit and miss, etc. Still, what am I going to run? OS X?
Also, not to sound like a fanboy, but why would you want Android on your iPhone? I can understand iOS fans' concerns about the UI on Android not being as polished as iOSes, but it seems like if you're going to use Android, using an Android phone just makes sense.