I downloaded the source and I think this would be a good place to integrate this feature...
I know you can download programs/scripts to do this as well, but none are in the repos and it really isn't much code to add.
Here's a screenshot of glade: [IMG]http://fileserver.mixtake.com/images/appearance_wallpaper_rotate.png[/IMG]
Let me know what you think!
Also I'm not well versed in C, so I'm not entirely sure how to implement the behind the scenes code to make this work... if only it was written in python :(