Currently, you can install android 2.2 on the iPhone "2G" (1st gen) and 3G. They are now working on the iPhone 4 and iPad, and have made good progress (openiboot running.) They have said that after they've ported android to the iPhone 4 and iPad, they will start on the iPod Touch's (not 3rd gen).
Past:
Android 2.1 ported to iPhone
Android 2.1 ported to iPhone 3G
Android 2.2 ported to iPhone and iPhone 3G
Openiboot ported to iPhone 4 and iPad
Future:
Android 2.2 (or 2.3) ported to iPhone 4 and iPad
Android 2.4 (Honeycomb) ported to iPad and iPhone 4.
Android 2.1 (dependent on CPU, GPU and RAM) ported to iPod Touch 1.
Android 2.4 ported to iPod Touch 4 and 2 (not 3rd gen)