I don't think that sunblock does much of anything to prevent tanning. The way I understand it, it blocks the UV that's bad but doesn't block whatever causes tanning.

I've got my biker's tan coming along nicely. I don't even think about it when I head to the pool. I figure it marks me as a triathlete in training.

I never noticed last year how much of a tan difference I get between the outer and inner lower leg.