bundler --without

bundler_test $ cat Gemfile 

source :gemcutter
gem 'thor', '0.12.3'
gem 'activerecord', '2.3.4'
group :db do
  gem 'activerecord-oracle_enhanced-adapter', '1.2.1'
end

bundler_test $ bundle install --without=db

Fetching source index from http://gemcutter.org
Resolving dependencies
Installing activerecord (2.3.5) from rubygems repository at http://gemcutter.org Installing activerecord-oracle_enhanced-adapter (1.2.1) from system gems Installing activesupport (2.3.5) from rubygems repository at http://gemcutter.org Installing thor (0.12.3) from system gems 
Your bundle is complete!

bundler_test $ bundle show

Gems included by the bundle:
  * activerecord (2.3.4)
  * activerecord-oracle_enhanced-adapter (1.2.1)
  * activesupport (2.3.4)
  * thor (0.12.3)