Speaking with a bit of experience (electrical engineer who has designed embedded systems similar to the PS) the cooling of the early units is definately marginal, and dust buildup WILL overheat them. But I have been running my fat PS3 almost continuously for months without worrying about it. I have it sitting on edge on top of the desk so it has no restrictions to airflow. If I hear the fan kick up to high I just clean it and that takes care of it.
I am more concerned with turning it off and on because temperature cycling is more likely to kill it than continuous operation. The power consumption is more an issue to me than the possibility of the unit failing. But hey, everything fails sooner or later. It's lasted me 3 years now so I have gotten my money's worth if it quits tomorrow. And I do back up my game daily.