It's getting too slow, since it's one of the earliest MBAs and I'm doing a lot of developing work which requires running virtual machines.
I had a look at the latest MBAs, but I don't know if they are as Ubuntu compatible as my current MBA.
If you're running VM, it should be perfectly fine. I read about some dual boot issues but I think that can be resolved.
I'm not running Ubuntu inside a VM, I'm running Ubuntu currently as dual boot. Just keeping a ~10GB OS X partition for eventually firmware updates. I want to do the same with the new MBA (if I buy one).