Well, it's obvious that the first two guys to jump in know nothing about what they're talking about. Of course I have experience and training, yeh, I went to school for this stuff. Sorry!
OK, so the first problem with fans is that they pull air from the sides. That means a fan pulling air through the cooler works less effectively than a fan blowing in. Many of the guys here would say that a pulling fan has a larger "dead spot" in the center.
The second problem relates to the first, since the fan pulls air from the side the only part of the cooler getting good airflow in the "pull" configuration is the top. Not the bottom, where the CPU is. Bad idea.
It's such an incredibly bad idea that Intel, that's the company with all the funky engineers, specifies their own designs, all of which blow into a cooler rather than sucking out of it.
Now there was this cooler maker nobody remembers, I think they're still around, called Alpha. They thought pulling was better, but realized that fans pulled air from the sides. So they installed a shroud around the top half of the heatsink, so that air had to be pulled from the bottom. A funny thing happened: Even though the sinks were well designed and had a lot of surface area which optimized cooling, further optimization was found by REVIEW SITES who reversed the fan, to blow inward.
This fan shroud is really obvious to dumb mechanics, as many hot rods have been found to overheat when you remove the fan shroud. Yet as previously stated, you get better results from pushing air.
One reason it's better to push air of course is the directional thing previously mentioned. But another reason is...air doesn't pull very well. It's a gas. It likes to expand. Pulling just lowers the air pressure.
So it's easier to reach the base of the cooler from a push configuration not only because it's directional, but because the pressure also helps it reach the bottom of the cooler.
All this has been discuseed numerous times of course. And there's other reasons why push works better, such as the air being cooler as you move away from the board. But when it comes down to it, you could simply search the forums and find this all explained in more detail in old posts.
And then you'd also find guys who were really good at arguing for the pull configuration...loosing. These modern geeks are too soft for that kind of debate.