Make the desktop report the OS X version I have to maintain several different versions of OS X for testing, and it would be nice to know at a glance which version of OS X is on a machine.

I've previously created custom backgrounds with the version number, but as the number of machines increases this is becoming difficult to maintain.

Is there a utility that can display the OS X version on the screen at all times, or automatically change the background image to a generated image containing the OS X version on each boot (or, I suppose, when run manually)?
Running sw_vers -productVersion will return the OS version.

You could then use something like GeekTool to output the result of a shell script to the desktop.

GeekTool is having issues under Lion right now, but the author posted an 'experimental' build for Lion. on his Twitter page.