Guys,
Hope you don't mind me posting here, but I have been having this problem with the normal AutoCAD 2010.
http://discussion.autodesk.com/forums/message.jspa?messageID=6166668#6166668
This has been happening since the ribbon was introduced in the 2009 version and was hoping it would be fixed in this version.
It defiantly has got something to do with the 64-bit version and I was able to reproduce the problem every time.
I raised a support request and was asked to reinstall .NET 3.5 and AutoCAD, but with no luck. So they sent it on to the developers for further testing.
It went a bit quite, so I asked for an update and they came back to me stating that they where unable to reproduce the problem in next version. Which I presume is the service pack.
Using the 'CLASSIC' workspace at the moment, which is unaffected.