Unfortunately, I've found the standard VirtualBox guest additions don't offer the graphics support to run compiz on Ubuntu 11.04, so testing Unity didn't seem possible. Thankfully, I stumbled across this post (Portugese) which shows you can install guest additions directly from the Ubuntu repositories:
With a freshly installed copy of Ubuntu 11.04 Beta running in VirtualBox, open a terminal window and type the following:
bashsudo apt-get update; sudo apt-get install virtualbox-ose-guest-utils -y; sudo reboot
Update: Also make sure you've enabled 3D acceleration in the guest machine's settings.
This line will install the up-to-date VirtualBox guest utilities, and reboot the virtual machine. You should now find the new - and very impressive - Unity interface ready to try out.