Does SMC Fan Control work on your 21.5" iMac? I have a base model (the latest one) 21.5" iMac and SMC fan control doesn't seem to work. It simply reads "0 degrees C".
Have you used the hardware test to see if you have any faulty sensors.
I don't think its that, because I installed it on a friend's iMac (that is identical to mine) for him and it didn't work either. The chances are of both having faulty sensors are slim. Oh, and when I installed SMC fan control it came up with a message saying "SMC fan control has not been tested on this Mac".
Why would you want to use SMC Fan Control in the first place? My mid-2010 21" iMac hardly gets warm to the touch.
Install iStat widget if you want to see your temps.
http://www.apple.com/downloads/dashboard/status/istatpro.html