Anyone doing insanely important backups will have tape cartridges, but for less important ones, I can see external Blu-ray drives burning those 400GB and 500GB disks (I think... I'm not sure... that the 400/500 disks are compatible with existing Blu-ray players... anyone know?) in smaller professional venues, though.
Yea, they are compatible with existing ones, at least with most of those (just read article that said PS3 can use those 400gb disks).
You can put blu-ray to Mac Pro if you really need it. Or just buy a PS3, it's one of the best BR players nowadays (Same kind of BR player would cost 800$).