ok, i just got off the phone with CRA. As i never lived in Canada before May 2010, stayed only couple of weeks in May to help my wife settle in Canada, and continued to maintain the rented place in US, and have no children, i become resident only in August 2010. Just because my wife started living here from May, i am not becoming a resident - automatically. Thanks.
That's interesting! The advice you got on the phone contradicts the advisories that CRA publishes.
As of May (not August), you seem to satisfy two of the three main criteria declaring someone a Canadian taxpayer. You have a dependant in Canada (your wife), and you presumably have access to the same home she occupies throughout the year. So May would seem to be the month you became a taxpayer.
CRA is notorious for giving bad advice to some taxpayers, and then covering themselves by saying you legally are not entitled to rely on their advice; you must consult an independent tax adviser.
I would advise you to try to get their advice in writing for your file. Some day in the future you may be audited, and CRA will want to at least charge you interest on unpaid taxes on foreign income between May and August 2010 (if not levy a penalty, depending on the auditor`s mood)-- but their letter advising August instead of May may help you negotiate a better deal.
If they won`t put it in writing, carefully note name and title of the person who advised you, so you can at least cite your ŕuthority`for using August as your landing date.