When I first got mine it seemed to ramp up the fans quite frequently after playing something like Drakes Fortune for half an hour. It's settled down since then though, and only maxes out when it first starts folding or has been playing a game and then is asked to do something more intense (like playing GT5P for an hour and then watching a 'busy' replay). One thing I did notice is that the fans tend to increase a little if you are downloading something, and playing a game at the same time.
I think if you leave it idle in the XMB for 15 minutes or so, and the console is still virtually silent, then there isn't a problem.