Silent
I have a Powerbook G4 15" 1.5 GHz right now and it is nearly silent. I am in college and use it during a 3.5 hour lecture that I have. It is silent the entire time. The fan never ever comes on for me in that situation. The processor is always set to automatic or reduced performance mode to reduce heat. Doing web browsing, instant messaging, taking notes in Word, and playing the occasional game (not full blown 3d gaming, just Snood like stuff) never activates the fan.
In contrast there are students in the class with PC notebooks that have loud fans and they are VERY noticable and ALWAYS on. The powerbook is completely silent.
Also, even when the fan does turn on it is a very very quiet one. It is so small it doesn't have the whir of a larger fan. It is more of a quiet clicking type noise. (Unless that is just because my fan is broken.)
Sorry such a long post, but I hope it helps answer your question.