"Unity 2D's goal is to provide the Unity desktop shell on
hardware platforms that cannot currently support Unity's OpenGL
requirements. Simply put, Unity 2D is a lightweight Unity interface
that can run smoothly on non accelerated hardwares. Unity 2D is not
a default in Ubuntu 11.04 Natty Narwhal and is very much a work in
progress. But from Ubuntu 11.10 Oneiric Ocelot onwards, Unity 2D
will replace classic GNOME as the default fall back desktop."