Ok, so I withdraw my previous definitions. Everyone is deconstructing me.

Dan, argued that social isolation and awkwardness is a key element to both being a nerd and a geek...although I would argue that it only applies to nerd.

I don't know anymore... wikipedia has a fair bit to say on the matter:

Pundits and observers dispute the relationship of the terms nerd and geek to one another. Some view the geek as a less technically skilled nerd. Some factions maintain that "nerds" have both technical skills and social competence, whereas geeks display technical skills while socially incompetent; others hold an exactly reversed view, with geek serving as the socially competent counterpart of the socially incompetent nerd, and call themselves geeks with pride (compare Geekcorps, an organization that sends people with technical skills to developing countries to assist in computer infrastructure development). Another view is that "geeks" lack both social competency and technical skills. Arguably, a nerd is a more self-controlled sort of person, while a geek can be something of a loose cannon--or at least more awkward in an obstructive way than a nerd.

Some regional differences may exist in the use of the words nerd and geek. Some claim that on the North American west coast the population prefers the term geek to nerd, while the North American east coast prefers the word nerd to geek (see Ellen Spertus's page on The Sexiest Geek Alive).

The word "nerd" refers to a person of "above-average intelligence" whose interests (often in science and mathematics) are not shared by mainstream society. By contrast, a "geek" is essentially a person who is fascinated, perhaps obsessively, by obscure or very specific areas of knowledge and imagination. A good example of this is found in an episode of the television show, "Married with Children." The protagonist Al Bundy complains about wearing glasses, saying he'll look like a nerd. His friend, Jefferson Darcy responds, "No Al you won't be a nerd. You're too dumb to be a nerd

Thus a "nerd" is primarily marked as having a high intelligence and is not necessarily more fascinated with one subject any more so than another. A "geek", however, is obsessively fascinated with particular subjects, but is not necessarily attributed an above average intelligence. Thus a "geek" has the compulsion and drive to learn vast quantities of knowledge about a particular field such as computers, or Star Trek trivia, without high intelligence being assumed. Some also hold that the reverse is true, geeks being those with high intelligence, and nerds being those who obsess over obscure subjects. More than likely, the main confusion between the terms comes from their common association with specific areas of knowledge that seem to require a high level intelligence (for example, mathematics and science). Thus a "geek" who was obsessed by the pursuit of mathematical or scientific knowledge, might also be classified as a "nerd" as society considers such pursuits to be intellectual in nature and as requiring high intelligence.

EDIT (added Friday; March 31, 2006 4:37 PM)

Adam just posted this very relevant and fascinating comment:

Interesting point (to me) in America at the turn of the century on onwards, the times when the travelling circus' of Carnivale fame roamed the country, the term 'Geek' actually refered to the person who would, as an act, consume bugs, worms, chicken heads and so on. Customers saw the attraction in the geek tent, or geek pit. The practice still goes on in certain circus', but I'm not sure they still call them geeks...

Amazing.