Canada will not become part of the U.S.A in the forseeable future.
First, their system of Republicans vs. Democrats is extremely well balanced and stable (too stable some say). Thus the addition of 31,000,000 Canadians would tend to give the Democrats a huge voting bloc to add to their constituency.
Naturally the Republicans etc., will make sure this will never happen. Nobody wants to rock the boat this way.
Next, by leaving our famous 'undefended border', in place, the U.S.A can point to the world and boast of their peaceful, non threatening ways. So Canada's independence is in the U.S.A's interest in world political circles, or at least on the streets of other countries around the world.
Last, they own us anyway. The U.S.A would be bothered and put out if they closed their borders for a week.
Canada would be on its knees if the U.S.A if this happened. We owe our standard of living and security to them and their military machine. IMHO this explains a lot of the endemic anti-Americanism because nobody likes to admit their status in the REAL world.
So Canada will continue to stand 'proud and free' (tee hee!)