From A Performer's Standpoint

We've previously skewered the perfectly idiot rock-"concert"-inspired notion of "audience participation" that would make it an OK thing for members of classical concert audiences to feel free to text to others, either directly or via Twitter, about a concert performance while the actual performance is ongoing. But violinist Hillary Hahn makes the case against such pop-culture inspired behavior much more eloquently:

I'm all for Tweeting and spreading the word, but not during performances. Between pieces, maybe, if you can stop when the music starts up again; while standing in line for the restroom, definitely; at intermission or on the train afterwards, definitely. The problem is that acoustic performers rely on the audience's attention and focus and can tell when the audience isn't mentally present. Your listening is part of our interpretive process. If you're not really listening, we're not getting the feedback of energy from the hall, and then we might as well be practicing for a bunch of people peering in the window. It's just not as interesting when the cycle of interpretation is broken.

RTW eloquent T here (scroll down a bit).