Merriam-Webster is doing its best to be up-to-date with all of the latest in popular words. It's probably best that they continue doing so, since they make dictionaries. They recently decided to add around 100 new words to their Collegiate Dictionary, one of which being 'fanboy.'
I've never much worried about using a slang word that my spellcheck doesn't like (ironically, it thinks that spellcheck isn't a word, despite being labeled as such in my word processor), mostly because if I did, you guys would think I was some 55-year-old guy in a suit. However, it is nice to know that Merriam-Webster recognizes yet another great word that I use on a fairly regular basis.
The official definition according to their dictionary is a “boy who is an enthusiastic devotee, such as of comics or movies.” This definition makes me wonder if we'll see 'fangirl' next time around.