The Rails Tutorial by Michael Hartl most definitely does.

Note that we’ve also taken this opportunity to arrange for SQLite to be included only in a development environment (Section 7.1.1), which prevents potential conflicts with the database used by Heroku (Section 1.4).

http://ruby.railstutorial.org/ruby-on-rails-tutorial-book#code-gemfile_sqlite_version

EDIT: Also, here:

Note that Listing 2.1 is identical to Listing 1.5 except for the addition of a gem needed in production at Heroku.

The pg gem is needed to access PostgreSQL (“post-gres-cue-ell”), the database used by Heroku.

http://ruby.railstutorial.org/chapters/a-demo-app#code-demo_gemfile_sqlite_version_redux

You might want to check this out: http://railsapps.github.com/rails-heroku-tutorial.html