Bundler throws uninitialized constant Gem::SilentUI (NameError) error after upgrading to Rubygems 1.5.0
Solution 1:
Update to bundler 1.0.10 or above (gem update bundler). The issue is fixed in later versions.
Solution 2:
If you're a bit vague on what bundler actually is and don't want to learn then the very short answer to the question is:
gem install bundler