Sorry for maybe newbie question, but where does the network hash rate figures come from?
I like the graphs like here: http://bitcoin.sipa.be/
I do understand that clients accept block according to the difficulty which in turn is adjusted so that it's difficult enough to get blocks every 10 min.
I don't see how the current network hash rate can be determined down to the hour, I mean do nodes/miners report if or how fast they are mining? Or does sipa.be just plot the amount of found blocks per hour? But this would take already the difficulty into account, making it hard to tell the actual Ghash/s.