Gemfile

source "https://rubygems.org"

def rails3?
  ENV["RAILS3"]
end

if rails3?
  gem "rails",               "3.0.20.github11"
else
  gem "rails",               "2.3.14.github50"
  gem "actionmailer",        "2.3.14.github50"
  gem "actionpack",          "2.3.14.github50"
  gem "activerecord",        "2.3.14.github50"
  gem "activesupport",       "2.3.14.github50"
end

...

Gemfile_rails3

# Gemfile for Rails 3
#
# …but don't actually declare any dependencies here. Put them in `Gemfile`. This
# file is is necessary to generate a separate lockfile for the Rails 3
# dependencies.

ENV["RAILS3"] ||= "true"
eval_gemfile "Gemfile"