Users have reported an issue with their Mac when entering sleep mode. After installing the Mac OS X 10.6.3 update and attempting to put their Mac to sleep, the system freezes, necessitating a hard shut down (holding the power button until the system shuts down).
Apple Support Discussions user Starnerg writes:
For some reason my computer frooze when I tried to sleep it. I quit all applications as usual before selecting sleep, and for the 2nd time ever. The computer screen went black like it does before it switches the monitor off. However it never switched the monitor off. I had to hold the power button down to shut the computer off (which I have a feeling is really bad for the hard drive).
ASD use Dick M corroborates:
Fair amount of posts concerning similar issues after 10.6.3 update. I had 3 shutdowns but happened differently than yours but with similar results.
When facing issues after an update to the OS, one of the best troubleshooting steps you can take is downloading and installing the latest Combo Update package available from Apple's Downloads site. The most current is here.
More advanced users can try the Terminal-centric solution provided by ASD user Lolipale in this thread.