Wooohoo Ubuntu 8.04 beta works perfectly in parallels on Leopard. I had some problems with 7.10 from the iso and cd so instead of using the alternate (text based install that is reported to work) I downloaded 8.04. So far everything is running flawlessly. Parallels tools reportedly has problems running due to an Xorg change, but I haven’t gave it a try for my self yet and that post/bug was submitted a couple of months ago.
Check out the Ubuntu 8.04 features. It’s exciting to see the new apps, features, and versions in open source land that are all being pulled together to create a great user experience.