I have this thought that involves using a mac laptop*, an external drive or two, and RAID. Am I insane or have you heard of this sort of thing working before?
I'm checking Google too, but real people I know are always better information sources.
* Specifically, Dulci the 17" 10.4-running PPC powerbook that I treat like a desktop and never take mobile. I'll use Time Machine and SuperDuper! with the Leopard machines.
[EDIT: Yes, you can do it. But it's really super impractically slow, so I'm not going to do it.]