What virtualization program are you running?

If you are using VirtualBox you can run apt-get install virtualbox-ose-guest-utils. You'll also need to enable 3D support under Settings > Display.

Though Unity works as expected (awesome!), there still seems to be some other bugs with VirtualBox integration. A big one for me is that seamless mode doesn't work when running 11.04.

Well it would work if you disabled unity - since unity is rendering everything as 1 display and not multiple windows the way X Windows wants.

Just did this: it works.

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:

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.

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

VirtualBox 4.0.6 added support for Xorg 10.1 which is the part that was required to get Unity working.