I'm still experiencing this issue. Killing iPoint.exe and re-loading it fixes the issue for me, at least for a while. I'm almost certain this is an Intellipoint driver issue.
It's not an iPoint bug. It's an Alt+tab bugg. Alt+tab is part of explorer so it's an explorer bug.
All Alt+tab has to do is make sure that the list of applications is an always-on-top window, with higher depth priority than the currently selected window. That's it. For some reason iPoint sometimes causes Explorer to mess this up. If iPoint can mess it
up then the Alt+Tab functionality is too fragile and should be made more robust.
I could be wrong and it could be an iPoint bug, but it seems more likely that it is a Windows Explorer bug. Either way, Microsoft product, and it's shocking that they don't care enough to fix it.