gterzian is on the right track, however, if you later update your version of ruby then you'll also have to update your .profile to point to the new versioned ruby directory. For instance, the current version of ruby is 2.0.0-p353 so you'd have to add /usr/local/Cellar/ruby/2.0.0-p353/bin to your path instead.
A better solution is to add /usr/local/opt/ruby/bin to your PATH. /usr/local/opt/ruby is actually a symlink to the current version of ruby that homebrew automatically updates when you do an upgrade. That way you'll never need to update your PATH and always be pointing to the latest version.