Will my laptop overheat if turned on in a backpack?

with the screen, processor, hard drive and almost all other components
turned off while sleeping. the powerbook does not generate a lot of heat.
if you're really worried though (like me) turn it off, at least when you know
you won't use it for a while.

As long as your Powerbook stays closed and does not open while its in your backpack, it will be perfectly fine. I bring mine to school and its in a zipped up case, which is then put in my backpack and I haven't had any issue. When the laptop is in sleep mode, IIRC there is only a tiny amount of power used to keep the data in the RAM, which generates next to no heat.

What I mean to say is, as long as your laptop is asleep, you'll be fine. If the laptop is somehow on while its in your backpack, it could overheat.

Let me confirm what the others have written, Stripes. I have had my laptop on since I bought it in March. It goes in my backpack (or occasionally a case) and then is in sleep mode. The only difference I have is that, to enter sleep mode I use the OptionCommqandEJECT keys and wait for the screen to go black before shutting the lid.

Ice cubes in January produce more heat than a sleeping PowerBook...

So that you don't post back with other issues, I am going to say that there is nowhere that you need to go that you need to leave your pb on inside your backpack in sleep mode. It is a delicate notebook and you are only inviting trouble when you don't powerdown and shut it off. I'd rather see your post with a problem knowing that you had the issue you have at the time because you took the precautions to secure and protect your investment. Better safe than sorry is the motto.

The sleep mode for what I recognize it for, is when you have arrived at your destination and are using it, that you walk away from your office or work area for extended time periods and your pb is secured with a cable lock to prevent theft, like going to the bathroom, a meeting or lunch or something similar. Then again, you are probably indoors for these situations and there is no need for the backpack. Others have indicated you can do it, but what if you arrive at your next destination and it doesn't wake from the sleep because the hdd somehow was damaged. You'll be kicking yourself for not having shut it down completely.

] Does that laptop need to be off for it to be safe to carry in such a closed environment?

I use my PowerBook in this manner all the time and everything has been fine.

My previous PowerBook was used in the same manner for three years, before I passed it to a colleague.