What say you to 'the interception of a telephone call from a US citizen to a relative in Egypt at a number that was used on three occasions to call a telephone number associated with a known Al Qaeda sympathiser in Egypt, who had not committed any terrorist acts himself but made statements in support of those who did, which number belonged to a neighborhood cafe'?
I'll go 8 to 1 that we are listening to many, many more of the latter than the former.
Regarding your scenario: Given the threat we face, given that we've already been attacked, I'd support the government monitoring that conversation. During WWII no one would have objected to the monitoring of, say, telegrams between American citizens and "words only" Nazi supporters in Switzerland.
During war, it's important to find out who your enemies are. Someone who speaks out in favor of terrorist acts should certainly be listened in on.