We had a big discussion about this in our crew lounge the other day. More like some F/As were discussing what they would do about a passenger using their cell phone, with me listening in.
Anyways, if it were me, I would just ask the F/A, "Would you like us to say something to those passengers at the gate?" If he/she says no, then I'd just leave it at that. If they would like action to be taken, I would just go back and politely inform the passengers that our policy remains the same about cell phone use, and we apologize for the confusion and inconvienience. However I ask they keep that in mind for the next time, and please be more understanding with our flight attendants. They are just trying to enforce the policies as instructed by the company, and do not deserve insults. I'd refrain from making a PA, just so I could pay attention to the task at hand (taxiing).
I think a lot of F/As would just say "fine, whatever" and tell us after all the passengers have disembarked. It just depends though. Some F/As (often younger ones) will call the cockpit regarding every departure from the norm (like when they are hit on by passengers for instance). Others with more experience will handle it themselves and not bother us while in sterile cockpit operations. As long as they aren't creating a large safety issue, or creating an uncomfortable situation for the F/A, then it really isn't a huge deal on the ground (after all it does say in the offical press release we are approved). In flight, it would be a different issue, particularly on approach.