Extreme Fan Speed (Error Code: 4SNS/1/40000000:Tp0P-131.000)

My 20" 2.4GHz Intel Core 2 Duo Aluminum iMac was purchased on 3/21/08 and it was perfect up until about the end of November. Now upon immediate boot up the CPU fan runs up to 3500 rpm for no apparent reason. At first this problem would only happen every now and then, but now it does it every single time. I've tried several things like reseting the PRAM and the SMC and that would work at first, but that doesn't do anything now. I ran the Apple Hardware Test and I got an error code (4SNS/1/40000000:Tp0P-131.000) when the fan was running at 3500 rpm, but when I ran the Apple Hardware Test when the fan was running at normal speed no error code came up. I recently took my iMac to an Apple Store and they looked at it and kept it for about a week and they said they could not find anything wrong with it and told me it was "normal" for the fans to speed up every now and then when it needs to cool down, but why would it need to cool down when I just booted it up and have no applications running? They told me that it could be a logic board problem or fan problem and they ordered the parts, but did not replace anything due to them not being able to find a problem. This problem has become extremely annoying cause the fan is quite loud and I absolutely loved the silence of the iMac when it was brand new. I don't have AppleCare, but my iMac is still under the 1 year warranty and I'm starting to feel that the only way Apple will do anything about this problem is if I purchase AppleCare. So has anyone else encountered this problem and if you have were you able to get it taken care of cause I'm getting extremely tired of this fan noise and I'm in desperate need of a solution.

iStat Screencap
http://i34.tinypic.com/213oqx3.jpg