Have no idea... I tried all key combo's, and restarted....
I have used Parallels since the first beta release, and this latest version (3810) is the first time I ever encountered this....
My guest OSes are Windows 2003 Server, and Mac OS X Server 10.5.6.
EDIT: This is on a MacBook Pro Unibody (Mac OS X 10.5.6)
My Mac Pro (Mac OS X 10.5.6) still does it well running Windows XP SP3.... the key-cpmbo = COMMAND - ALT - ENTER