I've been wondering the same thing about the hard drive...
I also just purchased an iBook(and loving it!)...and have been doing lots of video. I used to defrag my drive back before OSX came out, but unsure whether it's still necessary in this day and age. I'm working on an iMovie/DVD project right now, and the project w/video is around 30gb, which I've been adding-to, copying, moving, deleting, etc. Was wondering how much that all effects the drive's long-term health. I got the 80gig, so I still have about 30virginGB that've never been touched.
But I've still got one of those Norton-Defrag-colored graphic visions in my head getting all chunked up as I move all these big files around.