I traveled on Frontier Airlines beginning on 10/30/2008. On my outbound trip, there was no charge for my one checked bag (which there should never be; after all, given the restrictions on what you can take through security, you HAVE to check baggage now). But on the return trip a few days later, they tried to ding me for $15 to check the very same bag! They claimed that I had to pay this surprise fee because they had imposed it for "travel after 10/31", even though I began my travel before that date. When I complained, and told them that I would never fly them again, the agent reacted by making me a "selectee" -- that is, by causing me to practically be strip searched as I went through security.
Airlines shouldn't rip people off for baggage, and they CERTAINLY shouldn't be allowed to use the security procedures to punish consumers for standing up for their rights! Don't ever fly this airline.