Seagate just announced a 1.5 TB desktop drive, with the enterprise unit sure to follow. Delivery of desktop drives should be in August. If we used 48 of these in our 5U JackRabbit unit, we would be able to provide 72 TB raw. A rack full (8) would hit 576 TB raw, or nearly 0.6 PB/rack.
FWIW: we have customers whom have requested the desktop drive variants. We see failure rates about the same as the enterprise/NL units.