I read a rumor - got to love half baked rumors - that Apple were thinking about introducing water cooling in the iMac. I assumed if I did a search I would find some bored DIY modder who had fitted their own custom cooling to their (2010) iMac but I found no one who had done this (and that's not to say there is a reason anyone should; the iMac runs hot by design, efficiently so; put another way, why would Apple sell Apple Care on something they anticipate will break down; they wouldn't). I did find a few other folks directing cool air at the back of their iMac (which I found makes only 2 - 4 degrees C difference to the reported internal sensor temps with Temperature Monitor, despite the case then being very cool to the touch rather than close-to scalding; I was alarmed at how hot the glass is - on my particular iMac - though that area of heat does not correspond to the discoloration bands, if I can call them that, on my screen).
Again, I don't want to put people off getting an iMac, I see no reason to be, this is a forum where people either share their bad luck stories or more productively offer advice on problems that turn out not to be problems at all; the iMac is a genius design, a wonderful thing to behold and own. I simply got unlucky and know statistically it is much more likely that I would have got unlucky if I had gone for another brand. Out of a bored tech's curiosity it made look at the design in some detail, though.
Ifixit's strip down pictures are excellent. I intended at some future point, when the warranty expires, to remove the DVD drive and fit two slot fans and channel some thin ducts inside the iMac. The DVD slot is of course thin, though; it is hard to see how that wouldn't introduce a howling sound from the intake and outake slot fans if they were run at anything but at an ineffectually low speed. I thought more so about designing a desk with flush mounted (invisible) fans to direct slow-moving cooler air up toward the iMac's intake fans, 'slow moving' so as not to strain the iMac's fans (though again my experiments with Temperature Monitor showed only a 5 degrees C drop in temperatures at best when combined with my fans pointing at the upper back of the iMac). This could increase the intake of dust, though. Then, and how about this for a crazy thought, I thought about strapping the back of an iMac's case with polished copper pipes and radiator grill attached to the stand along with a water pump, to make the case cold. At that point, though, not knowing a great deal about water cooling, I would be worried about the possibility of condensation? Also, I imagine a fairly large water pump would not be inaudible, which the fans I am using are.
I think Apple's design solution for the iMac is extraordinarily elegant and refreshingly quiet but I have some sympathy for people who ramp up the internal fans rpm a little on the 2010 model so that the temperatures when the computer is idle/light surfing look less like my PCs temperatures do when playing games on said PC. Whether running third party fan control invalidates your iMac's warranty I have no idea, nor do I know how durable those fans are in the long term. Unless you were unlucky and have definite problems, relax and enjoy.