I have to second the recommendation for a passport. They're $85, are good for 5 years, and you'll never have any problem with ID. For $17 a year I think it's more than worth it. 2 other points to keep in mind:
- there's the ongoing talk in the US about requiring passports even for Canadian citizens. Last I heard they were talking about implementing this next year, but...
- if the US decides for whatever reason (god forbid something serious happens again) that they are going to require passports for everyone entering the country they under no obligation to provide any lead-time or warning whatsoever. Monday a random photo-ID can be fine, Tuesday they can require the passport. And if you don't have one, they don't have to let you in.
I don't mean to frighten anyone who isn't planning on travelling with a passport, I just want to reinforce the fact that if you have one you don't need to worry about if they require it or not.
I think because we're so inundated with American TV, and we've enjoyed such freedom of movement in the past that sometimes it's easy to forget that we're actually entering another country.