The "Unity 2D" desktop is to provide a Unity environment without the need for OpenGL or any accelerated graphics drivers, but is built using Qt and QML. The goal is to have Unity 2D to have a close to the same look and feel as the normal Unity environment. Canonical found a need to provide this Unity 2D option especially with many mobile ARM-based platforms lacking the needed OpenGL drivers, or at least for being available "out of the box" since nearly all of the ARM graphics drivers are poorly-supported binary blobs.
Unity and Unity 2D will be found in Ubuntu 11.04, to be released this April. There's also a Launchpad page for this new Unity 2D project.
Hopefully this move to provide a non-OpenGL Unity experience will not diminish the chances of seeing the Nouveau Gallium3D driver in Ubuntu 11.04.