This XS-Drive toy looks useful. I noticed Compgeeks has them for $89.99 without a drive. Then it is just a matter of picking up any 9.5mm laptop drive and slapping it in there. DPReview of it

On the other hand, it does seem a bit silly walking around with two harddrives all the time. One for your mp3 player and one for offloading your digital cameras. So they have a combined version called the XS Drive Pro. I wonder how well this one works. Very little info out there on this thing and it looks like it is only selling in the UK right now. If anybody has one, please report back.