I am NOT an attorney, immigration or otherwise.
I am, however, a dual citizen.
I was born in Canada to a Canadian father and American mother. We moved to the US when I was 11 and I entered the country (as a minor) under my mother's citizenship.
Canada recognizes the concept of dual citizenship. The US reluctantly acknowledges that other countries recognize the concept of dual citizenship, but they do not really believe it it themselves.
When I was nineteen and registering to vote in a Presidental election for the first time, I called the Canadian Consulate. They told me that they would continue to recognize me as a Canadian citizen as long as I never signed anything renouncing it. They also said that the US would not be as accomodating, and that whenever I was asked if I was a US citizen, I should say, yes, and not volunteer anything further.
They were correct. Even though legally I do not need a passport to enter the US from Canada, I have learned to carry a US passport with me whenever crossing the border because on at least two occasions, I honestly thought I was not going to be allowed to reenter the US when I provided a birth certificate from Ontario, no matter how much ID I could provide showing that I have been living in the US for more than thirty years.
Since my Canadian citizenship is by birth and not by naturalization, I don't know if you will be asked to renounce your US citizenship if you take a Canadian citizenship. But the US does not acknowledge dual citizenship easily or with any kind of grace.
BTW, my experiences with the US border guards were LONG before 9-11. I can't even imagine what it would be like now.
