I'm also seeing this issue with a Windows XP host on the following;
X.Org X Server 1.6.1.901 (1.6.2 RC 1)
Release Date: 2009-5-8
X Protocol Version 11, Revision 0
Build Operating System: Linux 2.6.18-128.1.6.el5 i686
Current Operating System: Linux johnbaxendale.zenpriam.net 2.6.29.5-191.fc11.i586 #1 SMP Tue Jun 16 23:11:39 EDT 2009 i686
Kernel command line: ro root=/dev/mapper/vg_johnbaxendale-lv_root rhgb quiet
Build Date: 18 May 2009 02:47:59PM
Build ID: xorg-x11-server 1.6.1.901-1.fc11
Easiest fix seems to be to as others have mentioned - swap keyboards in System -> Preferences -> Keyboard to something else and then back again.
Highly irritating - can't see the exact cause, but it's happened twice today already and so it's something fairly common. Only seems to happen after having switched to VMWare and back out again, I'm not using full-screen mode so I don't think it's related to full-screen mode in my case.