On Tuesday Eric Brewer gave a talk on this white paper at FAST. In the talk (and probably discussed more in the paper) he proposed looking at alternative disk form factors. He seemed to be particularly interested in "tall" disks, say 2U, that had the same surface area as today, but with more platters, and potentially more heads.
Ah, going back to the beginning of micros, where drives were often full-height things that could easily break feet when dropped. The thing that worries me there is that having had a big full-height 9 gb seagate scsi drive back in the day is that you needed to give the drive about a hundred watts on its own to run it, with a spool up that sounded like a jet engine was about to take off. I know that motors have gotten faster since then, and platters have gotten lighter, but that's still quite a bit of mass to get up to speed.
The thing that interests me more is the idea of more read/write heads. Add a second, independent set of heads for read/write and you will decrease latency a good amount. Or, a second dependent set of heads which would not provide as much of a performance boost, but would still improve latency a good amount.
