Today I helped out at a Cambridge Rails Bugmash. Along the way I noted some changes...
Within Rails the bin directory has moved from vendor/rails/railties/bin/ to vendor/rails/bin. There are plenty of online references to the old location.
I had some problems with instructions which included references to using sudo to install gems. Since I was using rvm, I needed to not use sudo in each case.
If planning to do some bug mashing on Rails I would recommend ensuring that you had Postgresql, sqlite and mysql set up correctly in advance. Also some familiarity with gems and bundler will help.
Craig Krigsman and Dan Pickett were there too.