It doesn't take much to learn pen spinning or other forms of object manipulation. You'd imagine how fast you progress contact juggling by messing around with it while you're watching TV, reading a book, etc.
Ha! Pretty much everyone in my highschool was doing it back in the day. I'm still doing it now! It seems to be a generational thing, though - there aren't many people outside of my specific age group that knows how it's done. It's a difficult thing to suppress, too - I find that the first thing I want to do with any pen I pick up is flip it. Annoys the hell out of everyone.