Lots of posts on lots of blogs about the guy that used a USB hub, 4 iPod shuffles, and the RAID software in OS X to create an iPod Shuffle RAID (i found the first note of this at this post on Boing Boing). The full instructions can be found here.
While this is kind of cool in a novelty way, the more important thing to consider is that you could do this with 4 big USB 2.0 hard disk drives. Imagine 4 200GB+ USB 2.0 drives cobbled together with the RAID software -- I think you could get out of this kind of configuration with a $40 USB hub and 4 $120 USB drives, so about $520 (I guess I'm assuming that you already have a Mac running OS X).
It makes me think about wanting to set something like this up to ensure that all my MP3s are backed up. Then I think about why this couldn't be achieved with some sort of firewire hub. Then I wonder if the drives are technically hot-swappable.