I tried out a USB-powered cooling pad under my mini 9 the other day, mostly because it went on clearance REALLY cheap, but also because I noticed my Mini gets very warm when doing things like watching video on Hulu over a wireless connection.
Major difference! The cooling is quite noticeable!
It's also useful when I'm using my Mini 9 for Skype over long periods of time when gaming on my desktop PC.
That's another time when the heat builds up.
Here's the one I'm using:
I've modified the cooling pad by simply removing the two sliding panels entirely, since the Mini 9 doesn't need them and it reduces the footprint of the thing by quite a bit.
The fans push air up into the bottom vents. I even can feel a little air exiting the small side vent on the right of my Mini 9.
I'm getting ready to disconnect the annoying blue LED, and possibly make one of the fans switched as well.
With one fan it's still cooled well, and makes half the fan noise!