I've also successfully swapped out the HD in my 120 to replace it with another 20 gig. I'm waiting for the single platter 1.8" 40 gig to arrive, and I'll do it yet again. The driver, is a torx T5, which by the way, is also for sale at misticaudio.com
It is a very easy, straight forward process. The only thing which worried me was how very fragile the plastic piece on the side of the unit is. It's the part which has the play/a-b buttons on it. It's actually a seperate piece which detaches completely, so you have to be careful.
Can't wait for the 40 gig to be released ! I'm also planning on painting my unit soon too.