Dual citizenship and passports when crossing border

When I cross the border from Canada to work in the US, I declare to US Customs that I am a US citizen, and normally I show them my Virginia birth certificate as proof of citizenship. My first question is: will US Customs soon (effective Jan 1, 2006) require that US citizens entering the US from Canada present a US passport?

Second question: I am effectively a dual Canadian/US citizen: US by virtue of being born in the US, Canadian by virue of being born abroad to Canadian parents. Does Canada object if I have both US and Canadian passports? Does the US object if I have both US and Canadian passports?

The trouble date for passports to go into the US is not until 2007 when driving a car. Canada and the US do not have a problem with dual citizenship. Check out Rich Wales' web site www.richw.org/dualcit
You do have proof of your Canadian citizenship, don't you?

Canada and US both accept dual citizenship. When entering Canada, use Canadian passport. When entering US, use US passport.