I experience the same scenario as described in Heroku deployment issue when I try to deploy my Rails 3 app to Heroku and sqlite3 is defined in the gems file.
/usr/ruby1.8.7/lib/ruby/gems/1.8/gems/bundler-1.0.0/lib/bundler/runtime.rb:64:in `require': no such file to load -- sqlite3 (LoadError)
Any clue why this is? The solution defined in the ruby-forum works, I just wondered why.