I can't find any, and I'd be surprised if it exists (it would mean drivers written for most of the ipad's hardware). Are you sure you aren't thinking of ubuntu on nexus (which is available, and easy because android IS linux)?
If you ARE talking about the ipad, care to share a link?
I actually have android on my iphone 3G (as a "recovery partition"). But I was asking specifically about ubuntu; perhaps I should have been more clear.
And that android, BTW, is completely unusable - I just keep it there in case my 3G gets borked in a need-to-reinstall way (like it did two years ago).
If you ARE talking about the ipad, care to share a link?