As a Canadian I've had problems coming back into Canada from the US. I was asked if I could prove the kayaks were mine and not bought in the the US. It was a bit of a standoff at the time as I tried to politely and calmly (difficult for me) to tell the officer that, no I couldn't prove it, not here and now! They did let me go through in the end with a warning to have proof next time.

The next time I go into the US I will first stop by the Canadian customs side and fill out a "green card" and have it officially stamped. This is a good idea for any expensive item (camera, laptop, kayaks, etc.)

I've never had problems crossing into the US as a Canadian and from what I've heard neither do Americans crossing into Canada. It must all be part of the harrasment program governments have against their own citizens!!!