For Revit 2012 users affected by Microsoft .NET 4.5, a hotfix is now available. The original post describing this issue has also been updated:
The hotfix link is included below:
This hotfix corrects the behavior of the ribbon panels after .Net 4.5 is installed.
Make sure that Update Release 2 has been applied prior to adding the hotfix. Verify that the build number of Revit 2012 is 20110916_2132. If the build number is 20120126_0600, it indicates that Update Release 2 and the Memory Allocator Hotfix has been applied.