Here's some good news! The Kensington SlimBlade Trackball Mouse connects to Ubuntu 9.04 Alpha 4 with no trouble, and it reconnects automatically after reboots, which none of the other Linux distributions I have tested were able to do - including Ubuntu 8.10. That makes the SlimBlade a lot more convenient to use. Also, it appears that when the SlimBlade is connected, the built-in touchpad in the HP 2133 is automatically disabled, which I really like.
Correction. Sigh. The touchpad isn't being disabled when the bluetooth mouse is connected. It isn't working at all! Apparently I hadn't noticed, because I always had a USB mouse connected. I just checked the Ubuntu bugs, and it has already been logged, and will certainly be fixed long before the final 9.04 release. Sorry about the misinformation.