"Canada’s Transport Minister John Baird today announced that airline passengers who are travelling to the United States are now permitted to bring one carry-on bag with some small exceptions. This new measure replaces the temporary restriction on carry-on bags introduced as a result of the December 25, 2009 security incident on Northwest Flight 253 from Amsterdam to Detroit."
and from YYC.com
"U.S. bound passengers are now permitted to bring ONE carry-on items on board aircraft. Transport Canada now permits one small carry-on bag for airline passengers who are travelling to the United States. This carry-on bag must not exceed a size of 23 cm x 40 cm x 55 cm (9 in x 16 in x 22 in), which is equivalent to the size of a small gym bag. The carry-on bag must fit into an airport’s carry-on baggage sizer."
The details go further to say that you can have the one carry on and a small personal item like a purse or laptop. Air Canada has a good chart describing these new allowances.