If you're reading this, you've probably had failures trying to install therubyracer with ruby 1.8 on OS X.
The fix is... (drumroll)...
RUBYOPT=-rrubygems gem install therubyracer
For those who are interested in the reasons why this works: the extconf.rb file for libv8 18.104.22.168, upon which therubyracer seems to depend, uses Gems without importing rubygems (which is fine in ruby 1.9). Adding -rrubygems to RUBYOPT makes sure rubygems are pulled in first.