I always like that Pulp Fiction line: "That's when you know you've found somebody special. When you can just shut the fuck up for a minute and comfortably enjoy the silence."
I don't think a moment of silence is any excuse to pull out the cellphone. In fact, I think this highlights part of the problem we have with cellphones. We expect constant entertainment, action, and info. It's really rare anymore to just sit for a minute and do nothing -- and it could be that doing nothing is pretty important.