Same happened to me when I flew to FLL for the Oasis cruise. Because of computer problems my flight to ATL was late and I had to run to my connection. I was last in line for boarding. They started about 30 people in front of me and gate checked our carry-ons. Now, this is a common thing when flying on a regional jet and they load those bags last and then deliver them to us when we land at the gate. However, on my flight to FLL they loaded our carry ons into the cargo bay of the 757 and then it went to the carousel with all the regular luggage. I was not happy. First off I have a very nice leather carry on bag, and second they didn't tell me it would go to the baggage carousel so I didn't remove my important papers. I had the foresight to put my passport in my pants pocket. My netbook, cameras, cruise docs, and other important stuff could have easily been stolen. Thankfully it was there when I arrived in the carousel area.
And it does come down to the airlines not enforcing the rules. On any given flight over half the passengers seem to have at least two carry ons (and often they are larger than the template), when the rule is one carry on and one personal item. Sorry, but a personal item isn't another full-sized carry on.