David Kanigan has a question from The New York Times: Are phones off-limits at the table?
My immediate and emphatic answer: Yes. I suppose the "Depends" responses include people who are thinking of circumstances where someone is awaiting a message of great importance; e.g. "Was Jonah able to get on a lifeboat?". Even with that, however, there should be an explanation of the etiquette breach.
In general, phones do not belong at the table and should not be used in restaurants. They are intrusions upon conversation and erode the atmosphere.
I'm sure this is a minority viewpoint but while in a restaurant, I'd rather have someone nearby smoke a fine cigar than be subjected to their loud use of a cell phone.