The switch to Git happened one day before we open sourced on June 17, 2008, I remember that day well :) So yeah, the history of when we switched to Pylons is probably lost forever sadly. But yes it was right around when I started in March '07 that you guys started working on the transition.
Personally I like flask a lot and it's come a long way. Also, when spez and I moved on to hipmunk we started using tornado.