A real Mac stumper

Ok, I've got a Mac problem that even I can't solve. If anybody out there is listening (Gruber? Drunkenbatman? Rentzsch?), please let me know what you think.

The situation: I use a Logitech MX500 mouse. I have Exposé mapped to the buttons on the side; specifically, Mouse Button 4 triggers the "All windows" action, and Mouse Button 5 triggers the "Desktop" action. I do not have the Logitech Control Center installed.

The problem (repeatable on my machine): if I quit the Dock, then when the Dock automatically relaunches, Mouse Button 5 no longer triggers the Exposé "Desktop" action. The weird part: Mouse Button 4 continues to trigger the "All Windows" action, and the F9-F11 keys all continue to activate their respective actions. I can restore Mouse Button 5's intended functionality by logging out and logging back in.

A Mac software developer, with whom I filed a mistaken bug report, thinks that some other piece of software is stealing that mouse button away from the operating system before it can be assigned to Exposé/Dock. But I have no idea what piece of software could be doing that. I also don't understand why I have full Exposé functionality on initial login if some other startup program might be stealing buttons.

Things I have tried that don't help: removing the SIMBL folder from /Library/InputManagers/, disabling extraneous login items, removing extra USB devices, creating a new test account and trying the same procedure.

Mac gurus, if you could brainstorm for a second and think of what might be causing this, I'd be very grateful. I'll even publish your name if you figure out what it is, and recommend you as a Mac expert. System Profiler reports can be supplied to trustworthy individuals.

UPDATE: Ok, new data point. I just killed the Dock, which of course had the same effect as usual. So immediately after, I went into the Exposé system preferences and switched the mouse buttons around; i.e. I set Mouse Button 4 to "Desktop" and Mouse Button 5 to "All windows". Mouse button 5 works just fine, but Mouse button 4 won't trigger "Desktop".

So it seems as though this isn't mouse-button-specific, but Exposé-feature-specific. Killing the dock also kills the ability to trigger the Desktop Exposé action with the mouse only -- the keyboard is unaffected.

UPDATE 2: Another Mac user reports the same behavior. I wonder if we have the same software installed, or if this is an OS bug.