As they read it. In Quantal there will only be one Unity (3D). That idea was discussed at the Ubuntu Developer Summit that happened in May, but it was confirmed in report a bug on launchpad.
As for those who do not have enough graphs …………. will have to use the Mesa 3D driver LLVMpipe, which does the 3D calculations on the CPU. That same driver is used by Fedora with the Gnome Shell when graphics acceleration is not available. According to the omgubuntu site, it is because with Unity 2D you could not have all the features that its 3D version had, adding to developers the concern that their packages will not be broken for the two versions.
Sources:
http://www.h-online.com/open/news/item/Unity-2D-dropped-from-Ubuntu-12-10-Quantal-Quetzal-1669508.html
http://www.omgubuntu.co.uk/2012/08/unity-2d-removed-from-ubuntu-12-10