For Canadians, a valid passport is already required for those planning on entering the United States via Air, such as an international flight from Toronto to Seattle. However, a passport would NOT be required if you were to drive across the border to Buffalo and then take a flight to Seattle from there. This is as of 2008.

There is currently, as of August 2008, NO requirement to have a passport if crossing the border into the United States by land or sea. However, it is recommended, since it is the best proof of citizenship there is. For $20/year, it's not a bad deal having a genuine document that proves that you are indeed a Canadian.

When will Passports be required for Land/Sea Crossings across the US Border?

I've been hearing that this will "soon be a requirement" for years, and now, there is a set date of June 1, 2009 for when passports will be a requirement for land/sea crossings. It is to be seen whether this will be delayed or not, or whether they will allow some type of enhanced drivers license to be used (like they will allow for US Citizens crossing the same borders).

For now (August 2008), all that is required (for adults) is valid photo ID and proof of citizenship (such as a birth certificate, or citizenship card). Children merely need to show proof of citizenship (But photo ID, even if a transit pass, is good to have).

Passports for Travelling Canadians Conclusion

My advice? Get a passport now if you semi-regularly cross the border, beat the swamp