Unity2D has been removed from Ubuntu 12.10 repositories with today's package updates.
Systems that do not support 3D hardware acceleration will now be able to run Unity 3D using Gallium3D LLVMPipe and users upgrading to Ubuntu 12.10 will be automatically transitioned to the newer Unity3D.
Earlier at UDS for Ubuntu 12.10, it was decided that LLVMPipe support will be added to Unity3D. This was done to focus on one desktop shell instead of maintaining two separate codebases.