I just upgraded this site to use bundler and the heroku Bamboo stack.
The steps were:
1. Remove the .gems file, and create a Gemfile.
2. Patch the Rails 2.3 installation for bundler
3. Run heroku command:
heroku stack:migrate bamboo-ree-1.8.7
5. Commit all changes to git, and push to heroku.
I am really impressed with the ease at which this was done. Heroku continues to impress, and Bundler is the best thing to happen to Ruby since Rails.