Well I just test android on one of my old 3G with iOS4. Using some new runtime called "R12 Runtime" at iphodroid.com. This was just released couple hours ago.
Little choppy but runs like a champ using the micro sim from my iPhone 4 with a sim adapter. I can send/rec calls and wifi works. I can also surf web with the google browser. Man this is cool experiment.
The guy sergio mcfly is working on this so hopefully will be looking forward to try this on my iphone 4 hopefully sooooon.