I don't know if it is acceptable, but I certainly see it more. I think society has become more casual as a whole and people just do whatever they want now. Honestly, it does not bother me too much. Loud conversations on cell phones and unruly children bother me much more. Still, if I am at a table with someone wearing a hat, I mention it is not good manners. Usually it does not make a difference, but who knows? Maybe no one ever taught them.