If you are using Ruby 1.8.4 you should update feedvalidator.

This release is an adjustment to workaround a bug in Net::HTTP#post in ruby 1.8.4 (see http://rubyforge.org/tracker/?func=detail&atid=1698&aid=4782&group_id=426)

Basically net/http in Ruby 1.8.4 does not send Content-Type header by default.

edgar@lacaraoscura.com