This has always been something I was thinking of, why caint I create my project initially with edge rails?
There are always minor changes in edge rails that can effect creating your project from using the stable version. Using the way below eliminates that issue by creating your initial project with edge.
mkdir -p new_project/vendor
svn co http://svn.rubyonrails.org/rails/trunk vendor/rails
ruby vendor/rails/railties/bin/rails .
Now you have yourself a fresh project base to start your new edge rails app.