After only hinting at its existence this morning, it is now possible to install Unity 2D on both the latest stable version of Ubuntu (10.10 Maverick Meerkat), and the upcoming 11.04 (Natty Narwhal).
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!
Read more: DownloadSquad