As of Dec 31, 2006... you
WILL need a passport to enter or leave Canada-US border via
AIR TRAVEL
As of Dec 31, 2007... you
WILL need a passport to enter or leave Canada-US border via
GROUND TRAVEL (that is, crossing by car)
WEB LINK REFERENCE:
http://www.pptc.gc.ca/travel_tips/trip_planning_e.asp
In the mean time, you need your birth certificate or something to prove you have Canadian citizenship and a govt issued picture Id (drivers license or your health card).
So, birth certiciate + (drivers license or health card)... drivers license is better as most Americans look at the Health Card with a very confused look and some even doubt its authenticity because they have never seen it nor have anything like it.
I would recommande a drivers license and birth certificate.
Enjoy your trip!