I bought one a few years ago from harddrivesuppliers.com (the site is currently down so I don't know if they still sell them or even exist anymore). It was a nice boost from a 4200rpm drive (seriously, I can't believe how much laptop tech has improved in the past 5 years), but I put a 5400 rpm hard drive in another laptop - an ibook - a year ago and couldn't notice a big difference in read/write for normal tasks.
For the gains, and considering the age of the technology, you're better off getting a nice 5400rpm PATA laptop drive with more storage capacity.
A word of note though. I would consider straying away from the WD scorpio line. The 120Gb drive I purchased had a prevalent firmware issue where the drive head would strike the platter after a few seconds of idle, making a fairly audible *TINK* sound. It hasn't caused my drive to crash yet, but I barely use it to begin with. They apparently released a firmware to fix the problem on pc only, but after removing the hard drive (a feat of courage if you've ever attempted this on an ibook) to try to upgrade it with a pc, I was met with failure at every attempt. Furthermore, I contacted WD and they basically told me they had no plans to support a mac firmware patch. Very annoying, and I've chosen to boycott their notebook line because of it (but I just can't resist their desktop class drives...).
