Yesterday I boarded a USAir flight from Charlotte to La Guardia right after buying a Starbucks capuccino and carried it in the same hand as the ticket I was handing to the gate agent, who said nothing. Once on board the FA wanted to know what I and my 2 companions were doing with the drink, didn't we know the new rules. I just said we didn't know and the gate agent didn't tell us, maybe they need to train the gate agent better. She just got upset with that remark, and I saw her go talk to the captain about the 3 passengers with starbucks drinks. Luckily for us, I assume, the captain refused to back the FA, since she just ignored us.

There was plenty of time, and if the FA had just politely asked me to either discard the drinks or finish them in the terminal I would have been happy to comply.

I did some checking after the flight, and it looks like this rule is not actually a TSA rule, but the policy of most airlines who are not sure what the rule is about liquids bought after the security screening!