I looked up and found the issue titled 10.6.3 Mouse Curser Freezing and read about the issue that meny had, because I'm having it now with OS 10.6.8.
I have the latest version Safari and Firefox for PowerBookPro running 10.6.8. The mouse freezing all of a sudden for no apparent reason also makes all keys on the keyboard non-functional. The clock is still counting and the power button still brings up the "are you sure you want to shut down your computer now?" screen, but no keys or the mouse works.
Sometimes, closing the laptop, pausing, then re-opening re-enables both the mouse and keyboard. When it doesn't holding down the power key until it shuts down is the only remedy.
Pro, Mac OS X (10.6.8), 2.4 Ghz Intel Core 2 Duo, 4 GB