The WP article is helpful: http://en.wikipedia.org/wiki/Dunbar%27s_number
This fellow's blog article is linked from the bottom. Among other things he digs back into the academic papers to understand what the number means (and doesn't) and works to come up with some other useful numbers (some only backed by believable anecdote). I haven't finished reading it yet.
I'm pretty sure I got the WP link from http://blag.xkcd.com/