This device has been mentioned a few times here. I've now got 4 of them in the field, some being used purely as NAS, some as more general purpose servers. They're Red Hat certified, and (from my experience) will run many other Linux distros.
Just for general information, there seems to be a promotion on at the moment. I've seen them on sale from several places for around £190 (including VAT). Just to add to it, some of the places that have them up for sale are also offering a cash back deal from HP - £100. Which means that you get a quite capable small server for around £90!
The current version runs a dual core Turion 2.2 GHz CPU, comes with 2 GB RAM and a 250 GB hard drive.
A pretty good deal for anyone looking for a small server, NAS or similar device. I've currently got no need or justification for buying one, but I'm having to fight the temptation to pick one up at that price.... Makes just about every commercial small NAS look pretty expensive, not to mention underpowered.