Nokia N900 has had multiple previous Android versions porter previously in a form of the NITDroid project. Even the still very fresh ICS has been working on N900 for some time already. What it has been missing so far is hardware acceleration on ICS.
Video by Sanjeev Aassori R
OS image has not been yet released, but it should not be long.
I will release the image ready for u-boot soon. Together with the libs from the video… Thanks to sanjeev for testing and making the video - vakkov