Although I’m pretty right-wing (at least for this board, anyway), I don’t think a single north american nation (well, barring tiny Central American Nations) would exactly be a bad thing.
However, the only way it could work would be by a radical change in peple’s perceptions, and a belief that the fundamental identities of American, Mexican, and Canadian were compatible somehow.
Then, the latter two nations would have to petition Congress to join up as territories and then seperate states (which they wouldn’t like). And in ordre to make the whole thing work, we might have to abandon Judicial Precedent, or limit it, and switch to Code Law as a legal principle. The capital might get moved - who knows, and nobody would exactly like that, because they’d be sitting in COngress in Nowhere, Kansas or something. And since each nation has its own political parties and electoral methods, we’d have a very complicated voting situation.
However, the Amero is an incredibly stupid idea. You could make the official currency name the Dollar-Peso, but Amero is just as stupid as name as Euro. And dammit, I want to put cool presidents on it. Mexico has some cool people. The Texans will be pissed to see Santa Anna on the 20$ bill. 
Canada, of course, don’t have anyone worth putting on currency.
(Zing! Gotcha, Canadopers!) 
Seriously, though, this is al just kinda silly. Heck, I think Mexico would have an easier time assimilating with the US than Canada, even, jsut because Canada’s identity is basically, “We’re not Americans. We’re like Americans, but less energetic about it. Same great taste, less filling.” There’s certainly something nice about it and less likely to cause trouble but it also cuts out so much of what makes America great in the first place. And it makes it rather hard for them to join up, since they’d ahve to fundamentally give up what makes them Canadaian to do it. Quebec, of course, would be totally freaked out because nobody in America would give a damn about them.
But the biggest problem with the whole idea is that it would massively complicate things. Governing America is terribly hard. Governing a nation of 500 million+ would be very much tougher. We’re not China with its extremely center-oriented culture and heavy emphasis on traditional modes of authority passed down.