Unity 2D (the package is called "Unity-Qt") got an Ubuntu PPA a few minutes ago and it can now easily be installed in Ubuntu 10.10 and 11.04.

I've tried Unity Qt in VirtualBox and I must say I am impressed: it's not identical to Unity 3D but it's A LOT faster! Further more, because Unity 2D seems to have more elements from Ubuntu 10.10 then 11.04 (even though some new stuff from 11.04 is there), it already has Dash (the application launcher / file browser) - something that's currently missing in Unity 3D in Ubuntu 11.04.