I worked both in the US and Canada last year, while being resident in only one country. Am I required to report all of my income from both countries to both governments, or is there some rule or guideline about which income to report where?
Canada and the United States have a tax treaty, meaning that if you are resident in only one country, under most circumstances you would declare income only in the country of residence.
The raw information can be found here on the Canadian Department of Finance Web Site:
The IRS info is here:
The information is very technical. I would recommend that you speak with an accountant, particularly if you're talking large sums of money.
A less technical summary from the US Treasury: