I have a MacPro Tower that I bought 4 years ago that was running OS 10.5.x. I *just* finally got around to upgrading the OS (I've had Snow Leopard discs for over a year and never installed them), so that's what I upgraded to. I have two displays connected to the system: a 23" Apple Display and a 20" Acer.
I was running with both displays before the upgrade just fine, and after initially upgrading to 10.6.3 my mouse was easily going back and forth between the two screens, but just ran the upgrade to 10.6.8 and now my mouse is "stuck" in the Acer screen and won't move over to my Apple Display - and the Apple Display is the one that has all the open windows and menus.
I tried unplugging and replugging in my displays and restarting the problem remains.