Android 2.3.1 Gingerbread has been finally released for iPhone 3G, 2G and iPod touch (1st-gen). Android ports for iPhone 4, iPad and iPhone 3GS are said to be coming soon.

However this is a preview release only, and therefore it is not distributed via popular Android installer package called Bootlace. You will have to download the binaries and install it manually using the instructions posted here.
This is a preview release of Android Gingerbread (2.3.1) for Apple devices, I am busy with other projects for a few weeks and the other 3 active developers are working on porting openiboot to the iPhone 4, iPad and 3GS, so I thought I’d put a test build out for everyone to have a mess around with, please read this post fully before trying it.
This release is for iPhone 2G,  iPhone 3G & iPod Touch 1G and is ONLY available as a binary package.
of course, You need to have a jailbroken iPhone or iPod touch to install it