I would like to add a permanent title bar at the top of a workstation's Desktop window to indicate a machine's security level (Unclassified, Confidential, etc). I would like to do this as a GPO, if possible, on a windows 2008 r2 domain for both Windows 7 and Windows XP workstations.
I'd say your best bet is to make however many wallpapers you need for each classification and deploy the wallpapers via GPO. If you're unsure of how to deploy them, instructions can be found here. You could combine this with different themes to easily distinguish between them.
If you want to make a permanent title bar on top of everything, you'd probably have to write a custom explorer.exe replacement.
The background image idea from @MarkM is a good one.
If you want something which is more constantly on screen you can also look into using a custom color scheme for your workstations. I've used custom colors to indicate production / dev / etc on my servers on and off. It shouldn't be too hard to set up a GPO to do the same for your workstations.
EDIT - Setting the active window title bar color worked well in my environment.