I overclocked my 7200 RPM Maxtor to 10,000 RPM and saw with my own two ears an obvious increase in performance. I noticed that it was smooth and my games ran faster. I have no benchmarks to substantiate my ridiculous claims, of course, so you'll just have to take my word for it.. hahaha.
No, seriously. How do you overclock a harddrive? The only think I can think of is that you can overclock the PCI bus, thus overdriving the interface (for lack of a better term for it). I'm not sure you'd want to do that though.. That would be a pretty good way of burning out your drive, no?