I wouldn't count on that being true, now. As of June 1, 2009, all Americans over the age of 15 need a passport to enter Canada. However, I read that many border crossings were making exceptions for the first few months last summer because a lot of people didn't know about the rules, or didn't have time to get a passport (there was quite a backlog at that time and it was taking a long time to process applications).
I wouldn't count on being able to just use your drivers license now, a year after the rule went into effect. I cross the border frequently (live in Seattle but all my family is in Canada) and have always been asked for my passport, since June 1, 2009.