You should be able to use your natz cert to get back and forth if you are going in and out of Canada in a car (till Jan 2008).
Use the original, not a copy.
I did this by air, last November (2 days after I naturalized). I didn't use the cert going into Canada (I'm a Canadian by birth), but I used it to get back into the US. I made a carrier out of two pieces of corrugated cardboard (with the corrugation at right angles) inside a zip lock back. I wanted *nothing* to happen to that certificate.
