We just got back - no passport required, only one person even really looked at our ID. Most didn't even ask for our kids'. FIL just came back from a US trip and had no problem.... July 4th weekend, no birth certificate.... no problem. He had a letter from a lawyer who has never actually met him before but was willing to state that FIL is who he says he is and would be in the states for such and such a time, and they took that.
I wouldn't bother with a passport - even US officials say that they would give Canadians an adequate time window to obtain a passport. But if it makes you feel better, get one. When we were asked for "passports", we told them we didn't have them but had everything else, and they said "your birth certificate is just as good".