I haven't worked there, but I imagine it's quite different today than it was in the first few years. They are more structured now, though still relatively flat considering the size. It's a team based atmosphere with team leads for each project. Teams rotate as needed, they aren't hard-line departments.

Scott Berkun worked there for over a year and then wrote a book about his experience. Probably as good of a resource you'll find, and a quick read. I reviewed the book here: http://www.poststat.us/year-without-pants/