It is not a feature that Sony put it, it just looks like a glitch. Here are some reasons as to why it is clearly not a feature...
1. If it was a special feature, why is it only in the 40gb models?
2. If it was a special feature, why does it not say this in the manual?
3. It sounds pretty loud, doing this could actually risk breaking the motor on the fan.
4. For it to actually clean the dust out, the fan would have to spin the opposite direction and I do not see this happening. It would actually blow the dust farther in.
It is no special feature, and I would not risk doing it as it could break your PS3 long term. Somebody should call Sony and ask though, find out what it actually is. I just know that I will never ever try this.