Have you upgraded RubyGems lately? Is your console suddenly filled with warnings like this?
NOTE: Gem::Specification#default_executable= is deprecated with no replacement. It will be removed on or after 2011-10-01. Gem::Specification#default_executable= called from /Users/chaffee/.rvm/gems/ruby-1.9.2-p0/specifications/thin-1.2.7.gemspec:10.
You may be showing signs of a new malady known as Warningitis! So far there is no cure, but doing the following will temporarily cure your symptoms:
gem update --system 1.7.2
This has been a public health alert. Please do not panic. SARS masks and iodine pills are not recommended at this time.