A similar thing started to happen to me when I set the system prefs to give me a few seconds before having to type in a password after the screen saver starts. Here's the story:
In the System Prefs, under Security, there's an option that says "Require password ___ after sleep or screen saver begins." For the blank, you can choose "Immediately," in which case once the screen saver starts, you'll have to type in your password to get back into your account. Alternatively, you can tell it to wait a few seconds before requiring you to type in the password. That's good so that if you see the screen saver start, and you're sitting right there and still using the computer, you can touch the track pad to wake it up from screen saver.
However, there seems to be a bug: When I set it to wait a few seconds before requiring the password, it prevents the computer from going to sleep correctly. First of all, it doesn't go to sleep after, say, an hour of non-use, like it should. And it has problems with the sleep invoked by closing the laptop cover. So, if I close the cover, the computer sort of sleeps (goes into a coma), but won't wake up, and I have to reboot. My solution was simply to set the "Require password ___..." option to "Immediately."
Hope that helps with this or similar sleep problems...
