I have been wondering way the average fan speed on my mac mini had increased from the ~1700 RPM that it was running when I bought it in the spring of 2008, to ~2600 RPM lately. Yesterday, I decided to both upgrade to SL and to swap the HD from a 200GB to a 500GB. As soon as i picked up the mini off my desk once it had shut down, I know the answer!!! There was a string of dust bunnies the shape of a "U" matching the air slots on the bottom of the mini. Duh!!! Okay, from now on, when the fan speeds start increasing, clean the air intake slots at the bottom. BTW, I used a can of compressed air to clean out all the dust inside the mini and it is now happily humming at ~1700 RPM again.