I've always liked Canadian folding money more than US... colorful and regal and all that.

If you can resolve the 50 cents on the dollar problem I'd vote for letting the Canadians join the party... and using their purdy money too.
Seriously, with NAFTA and given how close our governments are already... would there be any real advantages to combine (besides the wait getting accross the border)? Perhaps if Quebec really does decide to go its own way there might be advantages for the rest of Canada to think about such a thing, but people tend to become attached to their sovereignty, and build up loyalty to the idea of a nation... I just can't imagine Canadians giving that up (even though the Quebecers seem to be willing to).