I'm using a Windows XP desktop running inside Virtual Box under a Gnome Linux (Ubuntu 8.04). It's quite handy to dedicate a virtual desktop to the Guest, or each guest if you have enough memory for more than one.
Using my default Compiz keyboard I can jump into the guest using <ctrl><alt><arrow>, if the guest is in full screen mode then I get put straight into Windows and can type away, use a mouse etc.
Going back to Linux is a little more difficult as Windows throws away the Compiz key shortcuts. Howevever if you preface the Compiz shortcuts with <RightCtrl> (assumes you are using the default VirtualBox hot keys) then you can jump backwards (or fowards).