Well, it's yet another etiquette thing for the BBS to tackle. I never bothered to have an opinion on this until now, because my mom is on this streak where she's calling me out on using my smart phone in front of her.
My stance: I don't think it's rude to be on your phone while keeping company as long as you restrict your phone usage to about the same amount of time that you'd otherwise be using to sit there spacing out, not interacting anyway. I should get the same amount of socially acceptable non-interaction time as a version of myself without a smart phone would get. And I should be able to use that time as I see fit.
(You can't prove I'd be doing anything to acknowledge your existence right now if I didn't happen to have this phone so leave me alone, Mom!)