You have some interesting points Storm, but I'm still not conviced.
Firstly, in order for for a heatsink to be cooled effectively, it needs to come in contact with as much air as possible. Using a fan in reverse results in air taking the shortest route to the fan, thus the air *avoids* contact with the heatsink, especially at the centre where the temps are greatest.
By blowing air onto the heatsink it is possible to maximise contact so that heat is transfered to the air and then drawn away. This is why all heatsink manufacturers have their fans blowing air onto the heatsink.
Now imagine trying to cool your hot cup of coffee down so that you can drink it without burning your lips. Do you think that sucking air away, rather than blowing on your coffee, would have much cooling effect. No I don't suppose it would.
If you take a look at products such as CoolerMaster's new Aero 7, and GlobalWin's CAK4-88T CPU Cooler's you'll find they have tried to maximise the amount of air directed towards the centre of the heasink (plus using micro-fins to maximise air contact), and both have proven to be highly effective, and relatively quiet, coolers.
However, thats not to say that it isn't possible to produce a device that would work effectively with a sucking fan, but its design would have to be somewhat different to the heasinks that are currently used to cool CPUs.
Its true that most heat is removed from the CPU by conduction by the heasink, but the greater the temperature gradient at the centre of the heasink, closest to the CPU core, then the more effective the cooling will be. As already mentioned, reversing the heatsink fan will only reduce the temperature gradient, as the tendency will be to cool the outer edges of the heasink, which are already the coldest part.
Also, I don't feel that reversing the direction of a fan will have any effect on its ability to move air, so it will likely be just as effective regardless of which way round it goes. The important factor is how cool air makes contact with the heasink.
Although you've raised some good points (storm), I'm still not convinced (yet).