I had a phat, and I used to take a vacuum to the vents weekly. Occasionally, I would use canned air with it as well.
It YLOD'd last year, and I took it apart for the heat sync. Like Whoknows said, I was REALLY surprised at how much dust was in there.
I cleaned mine with canned air, and when it was too thick to brush off, I used a cloth with a bit of rubbing alcohol.