I remember thinking how impressive the roll-out of Subversion was. They reached some magic point where the majority of the development world just flipped and most everyone who've previously been on CVS switched in what seemed like an overnight transition.
Of course it didn't happen like that, but the perception of a sea of developers all collectively deciding to move on and knight Subversion the next savior seemed impressive at the time.
It's not so easy any more. First of all, Subversion is still a great SCM for the paradigm it embodies. It's unlikely to be out-gunned within its sphere any time soon. So any newcomers can't just out-SVN Subversion, like Subversion could out-CVS CVS.
Which means to topple Subversion, you have to bring a new paradigm to the table. The plethora of distributed version control systems seem to be that next paradigm. But it's not purely equitably "better", it's different. Better in many regards for many purposes, but not just better. Like SVN just felt better, period, than CVS.
So given all that, I think the Git move is even more interesting. That camp is competing not only to convince people that a new paradigm is appropriate for many things, but also as that it, one-out-of-many, should be the one to embody it.
I think they're going to get it. Killer apps makes or breaks any platform. With Github, I think the Git hub just scored one. Rails is going to be hosted there for the launch. Capistrano, Prototype, and Scriptaculous already moved there.
Go, go, Git.