Time Machine Question

Hey guys. Just a few time machine questions that i've been wondering about...

Will any type of external hard drive work for time machine?

Can you somehow use your other pc for time machine if they are all using the same network? via. Airport Extreme

And Will you be able to use time machine even if your external hard drive is connected to the airport extreme, or does it have to be directly connected to the mac?

All of your questions will be answered when Leopard is released this month.
Before then, you're pretty much only going to get conjecture and educated guesses.