For Canadians visiting the US - you need either a passport or a birth certificate. A drivers' license and Social Insurance Card is NOT SUFFICIENT. I realize that 99% of the time, a Driver's License is enough to wave you through but you need to have either a passport of birth cert. with you. About the "stamp in passport issue" I crossed the US-Can border around a dozen times this summer and it was never ever, ever stamped on any trip. So save yourself some hassle and keep a passport with you.