The world is moving more and more towards separating storage from logic, so you don't need a lot of CPU power attached to each disk; on the consumer side there are cheap NAS devices, but they (a) generally don't allow you to install your own code without major hackery, and (b) aren't designed for server environments.
I think this is a good idea, and I think it is very doable with relatively little up front investment, given what sorts of small linux-capable boards are available now.
If you put more brains on it, for example something like BackupPC, there would be a market for that also.
The world is moving more and more towards separating storage from logic, so you don't need a lot of CPU power attached to each disk; on the consumer side there are cheap NAS devices, but they (a) generally don't allow you to install your own code without major hackery, and (b) aren't designed for server environments.