I took my PS3 outside and used the leaf blower. I used the stream of air indirectly to clear the cloud of cat hair and dust, and to avoid overreving the fans. My PS3 was slightly quieter afterwards in Folding@home.
How many fans are in the 40 GB non-BC model? I can only see a large blower through the grille.
edit: I see no harm in the fan test. The fans are running at maximum rated speed for a short period of time, producing maximum airflow with maximum noise. There's no harm in doing it every once in a blue moon. Ejecting the dust back into the room might not be a good idea though. One commenter suggested intentionally overheating the 60 GB model to get the fans to run at full tilt by inverting the PS3, but that is a silly suggestion.