When I think of the distinction between geeks and nerds, I think about my friends who exhibit the symptoms of being either one, and I believe the difference comes down to the social aspects of the two. Both geeks and nerds are super knowledgeable about their area of interest, but a nerd can't turn it off. A geek is revealed only when they're engaged in their area of interest (think sci-fi convention, NHC, socializing with other geeks/nerds), or asked by a non-geek for information, while a nerd is constantly "on point", engaging in their passion at inappropriate times, in inappropriate ways with inappropriate depth in most social situations, and have nothing else to offer, socially. And I think you can exhibit signs of both.
I tend to vacillate between the two, but hang out mostly on the geek side of things. If you look at my house, my car, and watch me at a party, you'd likely never know I was a homebrewer. A few t-shirts are as far as I go. My garage, well that's a whole 'nother story, and on Facebook, where one is supposed to randomly share aspects of themselves, by nerdiness is apparent.