I've done it, however, I cut a hole in the side of my case directly above my cpu and draw cool air from there. Running a duct to the front can be quite a distance and you're going to a) greatly impact the airflow for the rest of your components, and b) have a large flow drop in your cpu duct due to the distance, so I'm not sure that is the best idea.
I have an opteron 170 with a stock cooler, slightly overclocked right now. With the duct on, I get about 50 C max temp when running prime 95 for a long time. Without was no contest....I passed 50C within minutes. Warm air from the inside of the case can't compare to cool air from the outside.
Now, I have an older case, before cases like the Antec 900 came along, which cycle a much larger amount of air than mine. You wouldn't see as much (if any) difference in a high airflow case.