I am also experiencing exactly this problem: keyboard completely freezes while showing the Spaces grid (on an Mac Pro, not a laptop). This started upon installing Snow Leopard. I am not using vmware. The mouse, which is plugged in through the USB port on the keyboard, continues to work. It has happened about 8 times in 3 days of using Snow Leopard.
I found that I can restart the keyboard by unplugging and replugging the keyboard USB.
If there's any connection, I've had a number of other problems with Snow Leopard, including numerous corrupted fonts (also reported by others), and at least one program (MS Powerpoint) that won't start at all apparently due to the font problems, which I haven't been able to fix.
I installed the new update (10.6.1) today. All these problems remain, including one keyboard freeze after the update.