Right now 18 Nov 2007, One needs a photo ID ( ie photo card driver's licence ) with your birth cirtificate. or the best is to have your Passport.
The US government and Canadian government are still in talks of what is required. Ontario is in developement of a high tech Driver's licence.
But as things are if you walk or drive to enter the US you will need a Passport as of 31 December 2007.