Yeah, I tend to be very picky about what I regard as a nerd. Nerds are specialists in knowing a lot about a particular subject, or have a strong passion towards general or specialized knowledge acquisition often to the point of being obsessive over it. (the way earlchip puts it is a prime example of how I would distinguish someone as a nerd, and I would always use such a term exclusivley in an endearing manner)
The type of folks that are equally obsessive, but over taking matters that are definitively trivial far too seriously and require little to no actual talent/knowledge to actually be a specialist in, I tend to refer to as 'dorks', and they are the type who are most often referred to as nerds in mainstream discussion. (I would put any of the folks warped described in this category over 99% of the time. (both of the complete douchebag, and non-or-at-least-less-than-douchebag variety)
Geeks on the other hand, they obsess over the process itself regardless of the subject matter in question. For them, it does not matter what they know, only about how they know it. It is the type of person who enters into pretty much any type of subject with a programmer's mindset, and while not all geeks are programmers, almost every single programmer out there is in fact a geek. He is exactly the type of person who would be really anal about providing a objective distinction between the definitions of various terms that are usually either clumped together, or used interchangeably to describe a large group of people far to generally. (You know, like exactly what I am doing right now)
Don't get me wrong though, the subset involving interest in gaming can be applied in all three cases. The actual distinction between gaming-nerd and gaming-dork is definitely a very subtle one, but a call most decent folks can make on gut instinct.