There's a thread on it, search for rotate. It rotates the entire x-server any of 4 ways. (I don't know if you can do mirroring or not...) You can switch via a status-bar applet, or (my preferred way) by powerlaunch.

The rotation is handled surprisingly well, actually. The full-screen keyboard is broke hopelessly, but the HWR and stylus-board work tolerably. All apps ought to be full-screened (the wide panel on the left is now subtracting from the 480px side), but most work fine.