I have a mid-2011 i7 Mac mini and I've replaced the HD with a (non-Apple) SSD. It's working great. When I use iStat Menus to view the sensor readouts for temperature/voltage/fan speed, etc, I note there is nothing listed for HD temperature. However, I have removed the Apple HD - as I said - so I can't tell you definitely without replacing the original HD whether Apple use a thermal sensor built-in.
On the other hand, I can assure you that the fan appears to work just fine. It idles silently, and turns the room into a wind tunnel when Flash is being played (all functioning normally, then :D ).