Is it me or has the development of Rails accelerated drastically lately? It used to feel quite slow, and now it's very fast. Is it because they switched to git? Any core developers here?
Next, around the time of the move to git, the existing Rails core team was shuffled a bit. Older members were retired (http://rubyonrails.org/core/alumni) and more active contributors (Josh and Pratik) were added to the team.
Then you've got the Merb merger which brought in Yehuda who works full time on Rails now, afaik.
If you look at http://github.com/rails/rails/commits/master you'll see commits from Josh, Pratik (lifo), and Jeremy, while http://github.com/rails/rails/commits/3-0-unstable shows commits from those three, Yehuda, and Michael Koziarski (who is still quite active).
Not to mention Rick Olson and all the patches from contributors, which now make their way more easily into core thanks to the streamlined contribution process and more active 'gatekeepers.'
So fresh blood, more time, easier to contribute, and exciting new ideas would be my guess.
Note that as contributor diversity appears to increase, LoC/primary contributor decreases.