[ame=http://www.youtube.com/watch?v=HQKfeDnMVQ8]YouTube - Android 2.2 Froyo running on the iPhone[/ame]
An Italian site by the name of iDroid, has managed to port the new Android 2.2 "Froyo" to the iPhone 3G only a short time after Froyo was leaked upon the internet.
The port is based on the iBoot method we've seen used already for Android on the iPhone 2G and 3G created by planetbeing for Android 1.6.
Interestingly enough, the port is almost 100% functional. Almost everything is working except WiFi and sound. You are able to send calls and sms messages, but I am not quite sure if you can receive them. I don't have the balls to try it out, but we'll try and get a guide up soon on how you can do this yourself. You will need to be Jailbroken of course and be familiar with SSHing.
It should be noted that there may be issues with the GPU which can cause the phone to get a little hot... as usual, be very cautious if you choose to gamble, tempting as it may be.
Until then, you can always translate the guide posted on iDroid.