A few people on the Apple discussion boards have reported a problem where upon updating to OS X 10.6.1 their systems will not wake from sleep mode. It is unclear whether the system fully powers off or just does not wake up, but regardless, it requires people to press and hold the power button to get the computer running again.
Reset PRAM and SMC
If you are having these problems, first try resetting the computer's system management controller (SMC) and the parameter RAM (PRAM). To reset the PRAM, restart the computer while holding down the options, command, P, and R keys all at once.
The computer will continually reset as long as the keys are held, and releasing the keys after a couple of resets to allow the computer to boot normally should properly reset the PRAM.
Resetting the SMC will depend on your specific computer. Here are a couple of knowledgebase documents that outline how to do this on various Apple systems:
Update computer Firmware
It is possible the affected computers have not been updated to the latest firmware that Apple has released. Be sure to check Software Update for recent iMac and MacBook Pro SMC and EFI firmware updates, and apply them if they are available.
Change the Sleep Mode
Since the computers are either powering off or are requiring a reset (which is effectively the same thing), you can preserve your work by forcing the system to go into a "deep sleep" hibernation mode. By default, the system will maintain memory contents in RAM so you can quickly wake the system and resume work. Only when laptop or UPS batteries get low will the system write the RAM contents to disk and effectively shut off.
You can force the system to always write RAM contents to disk, which will result in a slower sleep and wake-up process, but will also ensure that even if you have to press and hold the power button the system will restore the sleep image on boot and you can resume work. To do this, run the following steps:
Check your Security system preferences to see if "Secure Virtual Memory" is enabled.
For systems without it enabled, run the following command in the Terminal:
sudo pmset -a hibernatemode 1
For systems with it enabled, run the following command:
sudo pmset -a hibernatemode 5
This will not cure the problem, but should allow for the system to be resumed in the event of a power failure or the need for a forced manual restart after sleep. For more information on hibernate, read this.
Set computer to never go to sleep
The last suggestion is to never allow the system to go to sleep. For iMacs, you should be able to easily set this up in the energy saver system preferences; however, for laptops an extra step must be taken to prevent the system from going to sleep when you close the lid. This may not be practical for everyone, and will depend on how you use and transport your laptop.
To set this up, you will need to use a utility such as "InsomniaX" (program info here, program download here) to prevent the system from sleeping when the lid is closed. This will not hurt your laptop, but be sure to keep it from being placed in a confined environment where the back and sides are blocked off, since this may cause the system to overheat.