I'm running Ubuntu 11.10 on a MacBook Air [1] right now - much to the consternation of my IT department - and it works mostly-fine. The install was a bit fiddly and installing kernel updates breaks it, but both of those are due to needing to patch the kernel, which apparently is no longer needed for Ubuntu 12.04.

Day to day, the battery life isn't as good as it is in OSX, but other than that, it pretty much just works.

[1] (4,2 I think, but I can't remember how to check.)