You can just activate the reveal desktop mode in Exposé to hide your conversations when your prof walks by.
And when I said I would view it as a bug, it is because it is dangerous for PowerBooks to remain on when the lid is closed. Most PC laptops have fans that run all the time and various active cooling systems, whereas the PowerBook does not. If you try one of those hacks and leave your PowerBook running for about 15 minutes or so with the lid closed, I will guarantee you that you will either want to drop it like a hot potato or become worried that it will experience heat related damage. I realize it may be an annoyance when other laptops have that option, but the PowerBook isn't a Sony Vaio, it's a Macintosh PowerBook and it's just a different system. It's like buying a new car. Some cars, when the fuel gauge is on empty, you have a good 40 miles left, but on some cars when it is on empty, it really is empty. It isn't the fault of the car company that the gauge is different, it's just a different car. If it really bothers you, I understand your frustration, when I first switched, I was so frustrated with Apple and my PowerBook that I wanted to kick it and call up Apple to tell them why my old Compaq Presario laptop was better, but I eventually learned how and why things acted differently. If Apple allowed an option for the PowerBook to operate with the lid closed, would you then hold them responsible when there are large amounts of calls to AppleCare concerning their recently melted PowerBook?