See this website for a lot of Q & A from a Canadian accountant about this:
http://www.mnp.ca/en/media-centre/blog/2011/2/1/canadians-wintering-in-the-usThe first section of this website explains it quite well. I think this is where people are getting the "you are only 'allowed' 120 days per calendar year in the U.S.". If you don't want to fill out the form 8840 as described in the above website, then keeping your days in the U.S. to 120 per year will ensure that you don't have to fill out that form. I personally know Canadians who stay in the U.S. for as long as U.S. immigration rules allow (180 days) every year and they file their 8840's and have no issues.
We are watching where this goes, with allowances being changed to 8 months. The immigration department may allow this, but the tax consequences need to be closely watched. Will the IRS change their rules to align with this as well?
There is also provincial health coverage to consider. Some provinces in Canada are allowing residents 7 months out of province. I don't know if any allow 8 months yet. Our province currently allows 6 months.
