I replaced the drive myself (with a WD Black drive) on my late 2009 iMac. I was fully expecting to have to use
HDD Fan Control or similar to stop the fans from running. Turns out I didn't have to do that, since the drive supports the S.M.A.R.T standard, which can report the drive temperature over the SATA cable. In other words, everything worked as normal after replacing the drive and the iMac sensed the drive temperature just fine, even though I had disconnected the temperature sensor.
Takes a bit of fiddling to open it up, but I followed iFixit's excellent guide and there wasn't really any problem, not much harder than changing a hard-drive or memory of any other tight-spaced computer. My biggest worry was getting dust behind the glass, make sure you open it in a clean environment if you do it yourself.
EDIT: By the way, the WD Black is known for running a bit hot - I do get a few degrees higher drive temperature than the old drive, admittedly. But from what I read, that's normal with the wd black drives.