Looks like some interesting developments to help use KVM/Qemu on distributed storage is around the corner!
Just more back end to give easy migration and recovery on the front end, but it is well documented!
http://www.osrg.net/sheepdog/
Ceph looks cool, but not quite fully ready, (but in the most recent Linux Kernel), ok GNU/Linux Kernel
http://ceph.newdream.net/2010/03/rbd-rados-block-driver/