From the applyingtograd forums:

weakknees:
If you won an Olympic gold medal, would you mention it on an application to grad school? On the one hand, it's not relevant (maybe to kinesiology, but let's say you're applying to, oh I don't know, history) but then on the other it's a hell of an accomplishment!

apptake2
It really depends on the event. Because of medal inflation, it isn't necessarily a remarkable achievement any more. A friend of mine had two Olympic golds and was rejected from every school he applied to because it was a middle distance event (not very glamorous) and, on the second occasion, he failed to beat his own world record. Really, unless you're a world record holder and have multiple golds in a top-tier event you're not going to stand out from the competition. Bottom line: one gold won't hurt your application, but it won't help either. Having said that, I do know that for most top-25 schools, an Olympic medal of some sort is seen as the absolute minimum (along with an 800 on both sections of the GRE) for admission, but if you want to be competitive for funding you'd better back it up with a 4.5 GPA.