Well, the idea is to get the hot air out of the case and since warm air rises the theory goes its easier to put exhaust on top. You could change the air flow but then you still need a good way to exhaust the air out somewhere efficiently. Assuming you leave the front fans as is that only leaves the single back fan as exhaust. Not sure if thats enough. No one has really done a test yet and shown results of what happens if you have the upper fan as an intake. Non-overclocking setups with a single GPU probably won't have a problem but we don't know how bad a overclocked, dual GPU CF/SLI, with a bunch of drives rig setup will cool.
I'd certainly be interested in seeing the results of such a set up.