source 'https://rubygems.org'
ruby '1.9.3'
# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.0.0.beta1'Edit: Yes, I figured it out. In Rails 4.0, the default page for a new app is no longer served as a static file from the public/ directory. Instead, something magical happens, and the magic doesn't work on Heroku. That's no worry, though, as it only means the first app doesn't work. The other apps, which set root routes, should work fine.
They don't, though. I can't get the main sample app's CSS or images to load properly. Hmm... Bleeding edge indeed.
Update: You need to edit the default production.rb file to configure Rails to serve static assets:
config.serve_static_assets = true
To be clear, this is because the 'Welcome page' shows info about the server, so you NEVER want that accidentally leaked to the world at large by accident, so the route gets disabled in production mode.
It's not a big deal of course, as in any real app you'll quickly define a root route and be done with it. (I do consider it a feature, too, as I really like not having to remove the static file by hand.)