Just to add my report to this thread, I have the same experience with my bluetooth Mighty Mouse, of the buttons becoming unresponsive while the cursor movement still is normal. It didn't happen immediately after my 10.5.2 update, but in the last couple of days since that update it's happened twice. Once, fresh batteries appeared to fix it, and I assumed that was the problem. But then the following day it happened again out of the blue. I just stopped using the mouse for awhile (used my other Bluetooth mouse) and when I tried the Mighty Mouse again, presto, it was back to working.
So for me, this seems to be intermittent and I don't really know what the underlying problem is.