No, you're a Canadian citizen, but you'll have to start filing
taxes in Canada again. That isn't a big problem but bear in mind US citizens have to file tax returns every year until they die, regardless of where they live, so you will always have to file two (joint) tax returns while living in Canada.
Also there is the problem of the US foreign tax credit (which you will always have to claim on Canadian earnings on Form 2555), which I think in 2007 maxed out at $85,700 for single persons, not sure what the married limit is or what the 2008 limit is. If your income is over the limit you're subject to dual taxation.
They really don't like you leaving.
Steve.