Unity 2D now available for both Ubuntu 10.10 Maverick and 11.04 Natty
Unity 2D takes the form of a normal Personal Package Archive (PPA) that goes by the name 'Unity-Qt'. You can get it by simply adding the the Unity 2D repository, updating apt-get, and then installing it. WebUpd8 reports that the PPA is broken at the moment, resulting in some additional steps to get it working -- but just follow their guide, and you'll be running Unity 2D in no time!
Initial reactions suggest that Unity 2D is pretty darn slick, and a viable alternative if you can't run, or don't want to run, the 3D version. There's a video of it in action after the break.