i found this while i was searching "Recently I was at an event that was co-sponsored by Apple, and they had a PowerBook rep there. So I wen tand asked him outright, "Why dosen't the powerbook have this feature?
His answer was somewhat scary. He explained that the because the Apple laptop line is designed to do a majority of its cooling through the keyboard (if you lift up the keyboard, you'll notice the heatsink) this feature will probably be never included in any future release. If the lid is closed, and the machine is running as normal, it will cause the computer to overheat, because having the lid closed prevents proper closing.
So a word of advice, only use this if you absolutely have too. If you use it for a short period of time, you'll probably be alright. But if you're going to forget about it, I'd rather my computer just go to sleep rather than risk it overheating and cooking itself. More Info
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whether its true or not, im not taking a chance..