Nev -- I wish I had the solution, but all I have is the exact same problem...
Pbook 15" 1.67Ghz, about 11 months old, and has just started spontaneously going to sleep. Often it is wakeable after a prolonged delay, but sometimes it is not, and must be rebooted.
Very frustrating. I hope someone has some insight. How does one reset the power management module?