I've been doing a bit more research into gray hair lately (since like most men, I want to avoid it!). Here's a summary of a neat paper that proposes a "free radical theory of graying". In essence, the authors believe that just as damage from free radicals contributes to aging in general, damage from free radicals also contributes to gray hair.
To me, this theory rings true. I have seen other research which shows that smokers have more gray hair than non-smokers. I've also seen research where drug addicts show more gray hair than normal people. So it makes sense to me that oxidative stress is a major factor in gray hair. Certainly there is a genetic component as well, but at least the oxidative stress component can possibly be controlled/delayed.
For some more supplemental evidence, here is a recent study that looked at graying in a mouse model. For this experiement, the researchers applied a superoxide dismutase (SOD) gel (a topical antioxidant) on one side of the mice and then exposed both sides to oxidative damage. When the hair grew back, the side that was treated with the SOD gel was black, while the other side was gray.
This use of a topical antioxidant is in line with the recommendations put forth in the first paper I mentioned. The authors believe that proper "strategies will involve endo- and exogenous protection from oxidative stress-induced apoptosis by stimulation of the endogenous antioxidative capacity or by treatment with exogenous antioxidants". In other words, someone wanting to fight gray hair should focus on 1) getting enough antioxidants through diet and supplements, and 2) the application of topical antioxidants to the hair itself.
As far as I know, there are no effective topical antioxidant treatments currently on the market for gray hair. There are supplements that may claim this, but there is no clinical proof that I've seen. I bet that some of the bigger cosmetics manufacturers are working on something like this right now. Until then, getting a solid amount of antioxidants through diet and supplementation is the key.