Somewhat extensive notes here on the subject.
http://www.scit.wlv.ac.uk/~jphb/american.html
Ah, a topic of endless discussion in TEFL teaching rooms.
I have a theory that runs like this--British and American English are the most different in areas of vocabulary that developed in the late 1800s and early 1900s, when geography separated the countries a bit more than it does in the modern world. At that time a lot of new technology appeared (particularly household items and other various consumer goods) and words needed to be invented to describe them. And often different words were used on either side of the ocean.... Vocabulary from before this period tends to be more similar, since most of it was created before the colonists arrived in the Americas. And since WWII, increasing globalization and in particular the global mass media/entertainment (especially the worldwide popularlity of American film, and perhaps more important, TV) and now the internet are making the languages more similar again, and thus we're seeing fewer different words (outside of slang, perhaps).