Brad,
With all due respect to rkaufmann and Scott,Billings you are right to be concerned about the temperature of your Mac. Heat is the enemy of electronic components and excess heat will shorten their life. The issue here is a matter of degree, and just what represents “excessive” heat. Personally, the temps you report (and that rkaufmann indicates are “just fine”) would concern me. You say your iMac has “always run very hot”. Is it your impression that your Mac is running hotter now, or essentially the same as it always has?
If you take enough time to search through this forum, you will find hundreds (thousands?) of cases of 3-4 year old iMacs with heat-related failures which manifest themselves as frequent screen freezes, graphical artifacts (lines, tearing, etc.) on screens, kernel panics, etc. The problems reported by others seem to be frequently associated with dying GPUs, which, as your temps also indicate, is generally the hottest internal location, as reported by the iStat Pro widget.
It’s always a good idea to be aware of overheating conditions and take steps to mitigate them if they’re excessive. I would say step one is to make sure you put your Mac in a favorable environment – that “seems” to be good in your case, 25 deg. C, but make sure you have good air circulation as well as a clean, dust-free environment.
As blasphemous as some will find this suggestion, I’m also in favor of downloading one of the 3rd party fan control programs (SMC Fan Control, or iMac Fan Control). In what seems to be a favoring of ambiance over heat/temperature control, Apple engineers have implemented fan control algorithms that keep fan speeds low (i.e., quiet) until temps get quite high. These 3rd party fan control programs allow you to increase these fan speeds above the Apple minimums and help drive temps down a bit.
Having said all this, perhaps your biggest culprit is whatever is causing your Mac to slow down. The cause of that is also likely to be causing your system to work harder and therefore run hotter. (Going back to my question in the first paragraph above, this would suggest your iMac is running hotter now than on day one) Finding the cause of this should be priority one. Definitely contact Apple. You’ve paid for Applecare, might as well use it.
Good luck, and keep the forum posted on whatever you may find.