I've gotten a few custom Rails 3 apps, as well as Opal app working on DH. All of which use bundler. Sometimes you'll run into weird errors depending on the app. Sometimes i'll have to store all my bundled apps locally with...
bundle install --deployment
Or sometimes I'll have to specify the Gem location if my app is using a lot of gems that aren't stored on my server system-wide, that I installed locally with gem install. I do this by adding this to config.ru:
ENV['GEM_HOME'] = '/home/[USERNAME]/.gems'
After this, things usually work. Hope this helps.