Wait a second. Can anyone post a link to either 1) a person that says they've adjusted the rotational speed of their hard drive or 2) a program that definitely has this feature (I assume such a feature would be listed amongst its features)?
The supremely excellent Hitachi Feature Tool
allows you to change the SMART seek acoustic setting of the drive, but I see nothing in its feature list nor have I encountered any option while using it that indicated it is capable of changing the rotational speed of the drive. The equally excellent Nero CD Speed
will let you slow down your CD drives, but couldn't care less about your hard drives.
I kind of suspect that not only is there no software to slow down hard drives, but that running a hard drive at a variable speed would somehow interfere with the integrity of the current system by which hard drives work. Otherwise, I'd expect that battery-limited devices like notebooks would have additional power modes for their hard drives besides "on" and "spun down". Similarly, hard drive firmware seems to lock out access to the drive until the drive is "spun up" when booting the computer. I don't know why a fixed rotational speed would be vital to the operation of current hard drives, but it sure seems like it is.