Unity is built using Compiz and requires 3D acceleration; a lot of people have issues running it in a VM. There's a Unity2D project built using Qt, but I don't know if it was mature enough at the time of release to make it in as a fallback.
I've just tried it within the latest VirtualBox today and it works wondefully well. The install was simple and efficient, and it integrated nicely from the get go (with being able to use mouse and keyboard seamlessly).
I only had to install the guest extensions to get 3D acceleration and Unity to work.
You can install Unity alongside Gnome, and choose between them using the menu at the bottom of the login screen.
To do this, open Synaptic, go to Settings -> Repositories, and in the Other Software tab click Add and paste this line:
ppa:canonical-dx-team/une
Save your changes, then click the Reload button in Synaptic. When the new package lists are loaded, install the package called Unity. When you log out, you should be able to choose "Ubuntu Unity Netbook Edition" from the menu on the login screen.
