This is - how do we put it - more of a global problem than one just restricted to cheaters. Everyone these days is ignoring everyone else in order to pretend to talk to everyone else. It's a strange situation when four people together in a bar are spending their time silently and furiously messaging people who aren't in the bar, but you're not alone, buddy.
There is, however, a time and a place for this kind of thing. And when it comes to romantic relationships, there's a definite line that can be crossed. If she's doing it while you're trying to have a romantic picnic, or it's the first thing she grabs for in the morning instead of you, it might be time to literally and metaphorically pack things up. Clearly there's someone on the other end that's getting her undivided attention.