The guys at NexusOneHacks.net have managed to install and run Ubuntu in Nexus One. And best of all they have installed it in such a way that it will not destroy the existing Android system on the phone. This means that the Nexus One can still be used as a phone.
Not only have they managed to get Ubuntu up and running in Nexus One, they have also posted a video showing how you too can do it if you have a Nexus One.
Here is the video:
If you are brave or adventurous enough to try this, head over to NexusOneHacks.net. They have the full instructions on how to do it.