If I marry a Canadian and file a U.S. joint tax return this year, if she remains in Canada pending an immigrant visa, does she have to file a joint tax return in Canada?

There is no joint filing in Canada.  As a resident of Canada, she will file a Canadian return.  She will report her marital status as married, and will report your net income.  She will use a Social insurance number of 000-000-000 for you and add your name and address as non-resident.  She does not report any of your income tax return details.  The net income figure is to enable the software to calculate her deductions correctly.

You can file a joint US return. 

Very helpful, thanks!

You said ... "She will report her marital status as married, and will report your net income." and "She does not report any of your income tax return details."

- does this mean, for this case, the U.S. citizen's income would not be taxed in Canada?