For those not on Mac OS, it pops up a window that shows you all the Dropbox founders (and maybe some other engineers or something) and then sends a message about how they love working on Dropbox, and then some platitudes like "be yourself" and "love thy neighbor"... all animated between scenes, etc. like old Demo Scene stuff.
It only seems to work once so make sure you wait and watch the animation. I was going to record it to put on YouTube but couldn't make it happen any more times :-(
I find it weird that so many easter eggs turn out to be the credits for a piece of software. Shouldn't that stuff be proudly displayed front and center?
http://youtu.be/Yxz6D83Ljp0?hd=1