The top is not necessarily the best place for a fan, despite the fact that heat rises. A computer case is so small that there is little to no air movement caused by heated air rising above cooler air. Without fans, the entire case would be hot, top to bottom. The most important thing by far is proper air movement.
A computer case has so many passages, air restrictions, dead pockets, etc. that trying to pull air straight out the top leaves most of the case uncooled and full of hot air pockets. The most effective cooling designs I've seen have multiple exhaust points which work together to eliminate these dead spots. My PC-60 with the three stock fans turned on "high" along with a dual-fan Enermax keep it very cool.
The other porblem with a top fan is that it robs air movement from the rear exhaust fans which are the most imortant. Their main purpose is to pull as much air as possible from the main air flow across the cpu, motherboard and video card and get it out of the case before it runs across anything else.
Of course every case design is different and those which acually have air flow high on their list of importance (very few it seems) place fans where they work best for the paticular case. The power supply's position and size is a huge turbulance-maker when using a top fan. If the PSU were outside of the case, then the best design would be multiple angles of intake at the bottom and the same for exhaust at the top. For some designs, a top fan may work, but for most it's inefficient and can hurt the case's cooling capabilities.
-- Ah sh*t! sys64738 --