Well, all the news programs here have been reporting for weeks that passports are going to be required but it seems that a state issued picture id plus a birth certificate will suffice - however, I think they're suggesting passports as the quickest means of entry. Also, they're saying that this could change to passports only at a time in the future.
I'm not sure why, after looking at the customs website, they're pushing passports only so hard over here.