If an individual was in Canada temporarily, worked, and received a T4, that individual files as a non-resident of Canada, not as a resident with an emigration date.  The situation in the original question in this thread was clearly that of a non-resident.  They had permanent ties in the US and filed a US tax return (presumably they were a US Citizen as well, but it doesn't really matter).  

Article XV of the US/Canada Tax Convention covers employment in Canada by a US resident (or vice versa).  If the US resident earns less than $10,000 Canadian through employment, they only report that income in the US.  Also, if they are resident in Canada for less than 183 days, they only report the employment income in the US.