I used to be completely 100% against any kind of cell phone use in the theatre. I was always the cell phone Nazi to my friends, insisting that everyone both SILENCE their phone AND turn it completely off.
...but I can now actually see some exceptions.
One of my friends I usually go to the theatre with is extremely respectful, but when his sister was 8-9 months pregnant, he would keep his cell phone ON, but completely silent during the show. That way, he could check quickly and quietly once or twice during the show just to make sure his sister hadn't gone into labor.
Also I have a relative who is a doctor and is often called in on emergencies. He does pretty much the same thing with his beeper, keeping it on complete silent as not to disturb anyone. He checks it unobtrusively just a few quick times during the performance.
Both of these people are avid theatergoers and I don't think a) having a pregnant sister or b) having a job where you can almost always be called in, is any reason to have to stop going to the theatre.
So, if I don't HEAR it or SEE it or NOTICE you checking, I don't see anything wrong with this kind of important cell phone use being someone's personal prerogative.
I still always turn mine on silent AND off.
[title of show] on Broadway. it's time. believe.