I use Ubuntu 7.04 on virtualbox beta 2 and it runs great. Just create a machine in VB (select "Linux 2.6"), install Ubuntu normally, then reboot the VM and install the guest additions. Of course, virtual machines have yet to reach the full graphics potential of their hosts, so don't expect to get any serious 3D acceleration, but otherwise I feel that as long as you give the VM 512mb of RAM it will run at native speed. Good luck!