USB 3.0 will allow one to set up a striped raid array in an external enclosure to get faster data transfer rates than what is possible with a single drive.
Raid-like technology could be built into hard disks, through use of multiple read-write heads arranged around the disk. Conventional heads sweep across 1/4 - 1/3 of the disk, so 3 or 4 heads would be the optimal setup. It would be easier to configure than conventional raid arrays, whilst being backward compatible with conventional drive controllers by simply not using the secondary heads.
It's interesting you would mention that. That is something that occurred to me a couple of years ago. I mentioned it to a friend of mine and he told me about some HD company that experimented with that concept a few years ago. As I remember, they used it briefly, but dropped the idea. I don't remember why; it could have been that the drives were more prone to failure, or perhaps the performance gains weren't justified by the increase in complexity. Whatever, the reason, the idea didn't last long. Personally, I think it's a good idea that has promise.