Somemore info
Post Sept 11, the Canadian GC holders, need a visitor-visa to enter US. However, there is no visa requirement for Canadian citizens (of any origin).
"Beginning on March 17, 2003, persons with landed immigration status in Canada must obtain a non-immigrant visa in order to enter the United States. Such persons must also have a valid passport from their country of citizenship.
However, this regulatory change will not affect persons with landed immigration status in Canada who are citizens of countries that have a visa waiver agreement with the United States. A visa waiver agreement provides for reciprocal visa-free entry into each other's territories. Information on these visa waiver agreements can be found at
www.amcits.com/waiver.asp or
www.travel.state.gov/vwp.html.
The United States has visa waiver agreements with the following countries: Andorra, Austria, Australia, Belgium, Brunei, Denmark, Finland, France, Germany, Iceland, Ireland, Italy, Japan, Liechtenstein, Luxembourg, Monaco, The Netherlands, New Zealand, Norway, Portugal, San Marino, Singapore, Slovenia, Spain, Sweden, Switzerland, and the United Kingdom (including its colonies, territories, and dependencies). Persons who are landed immigrants in Canada and citizens of these countries are not required to obtain a visa prior to entry into the United States if they are travelling for business or tourism and their stay will be less than 90 days. Persons with landed immigration status in Canada who are citizens of countries other than those listed above must obtain a visa from the American authorities before entering the United States.
Persons who have landed immigration status in Canada and who travel using either a British National Overseas Passport or a Hong Kong Special Administrative Region Passport will require visas in order to enter the United States. "
Source:
http://www.voyage.gc.ca/dest/report-en.asp?country=308000
Bob