You can still run Unity on any VM (or lower powered machine) by using Unity 2D. It doesn't ship with 11.04 (it will in 11.10, I believe) but you can get it like this:
It's in 11.04, you can just install the "unity-2d" package; you should only use the dailies if you're looking for trunk builds that might not be stable
sudo add-apt-repository ppa:unity-2d-team/unity-2d-daily
sudo apt-get update
sudo apt-get install unity-2d
Then log out, and choose 'unity 2d' from the login menu when you log back in.