Ahhhhh I though it was some sort of btrfs specific thing -- didn't recognize it as the 'md' in 'mdadm' (I've only dealt with software RAID before).

This is actually pretty interesting because one of the 'features' of the hosting provider I'm using is that they will software RAID your drives by default. Maybe btrfs is a better choice in that kind of environment if I don't have to undo the software raid on every machine and btrfs will interop well without too much abstraction.