We had a weird issue this morning where the Archive options were missing in Outlook 2007. This is apparently a known issue with the kb2412171 December 14, 2010 outlook update which allegedly improves stability. It is hard to see how removing functionality, breaking send and receive and reducing performance when you switch folders comes under the category of improving stability and increasing performance.
Thankfully the solution is simple, just remove 2412171 from add/remove programs and everything should go back to normal.
Further details on the patch are also available which includes the known issues when installing 2412171.
Needless to say, we have unapproved this patch on our WSUS servers.