I'd be surprised if no one has experienced it yet. I edited and rendered a 35 minute video through iMovie a few nights ago and about 5-10 minutes in the fan became very audible. Not enough to be any sort of dealbreaker, but enough that I thought "what's that noise? Where's it coming from? Woah is that the Air? Huh, I guess it is."
Then I placed the Air in a spot that would be conducive to better airflow in and out of the unit and went about my business.
I have a 13" 1.86/2GB version now but will be returning it and have ordered the 2.13/4GB version. I can only assume that the 2.13 CPU will likely trigger the fans earlier, but with how I plan to use the Air I'm not very concerned that it'll happen frequently.