Many of the new enthusiest cases come with some type of ducting like that already installed. I modded my old case like that and it dropped my cpu temp by about 7deg C. Of course, my last case had terrible air flow to begin with......
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.
I have an old case and was considering (still am) changing to an Antec 900 but I quite like the thought of modding my old one just to see what I could get it cooling to while still making it look (nearly) totally standard and tubing was one idea I came up with.
I was toying with the idea of adding another intake fan but then thought about adding it but running a tube to it and to the cpu.. so there will be no restriction on air flow.
I have an Antec Sonata II case and it includes such a device. There is an adjustable duct system to draw air from the case rear to over the CPU cooler. In a way, the CPU cooler fan becomes an intake fan. So my case has two intakes - CPU fan and a regular 120mm intake fan near the front drawing over the HDD's - and two exhaust fans - a 120 mm exahust at the back and the PSU fan. My only concern is whether the rear intake chute pulls in warm air discharged from the nearby rear exhaust port. For that reason in my desk I removed the rear panel of the computer shelf and replaced it with a wide-spaced metal mesh as the desk support element. That way the warm exhaust air has someplace to go instead of being trapped back there.