I think Neubarth is probably right. Even with something like a fire I can't imagine they didn't have a distributed hosting model or disaster recovery site of some sort. I don't think they would be dumb enough to build a system reliant on a single DRM server either but then again I have seen worse!
If something gets corrupted and replicated to your other sites it can cause major problems and a big SAN can take a long time to restore just due to the huge amounts of data coming back from relatively slow tape.
Amazons own systems are probably propreitary like Google or heavily customised and designed from the ground up to be highly scalable and distributed. It might be quite a job to migrate the Mobipocket systems to the same platform.
I'm suprised they still haven't opted to provide more details on their holding page though.