The key limitation to growing rice in northern climates is not heat as much as it is day length. Unlike a lot of modern wheat varieties (such as those introduced all over the world during the "Green Revolution"), as far as I know, most (perhaps all) rice is photoperiod sensitive. That is, growth is affected by how much daylight it gets.
Here's a link to an older IRRI publication that says a lot more than I could here:
http://pdf.usaid.gov/pdf_docs/PNAAW647.pdf
I used to live in China, where I learned about rice grown in the country's most northern province called Heilongjiang (黑龙江, or "Black
Dragon River, which is a pretty cool name). Rice is being grown on the 45th parallel, which is actually further north than Hokkaido, Japan. In fact, the border of Vermont and Quebec is at the 45th parallel also, so it makes sense that rice can be grown in Vermont (as mentioned in a previous post).
Where I live in Manitoba, Canada, at the 50th parallel, a good friend tried growing upland rice (not wet paddy rice, but a dry field variety). Initially it grew fine, but then over time it became sickly and never reached maturity. Our guess is that it was becoming sick due to an imbalance in light-dark periods (i.e., because of photosensitivity). The funny thing is that, at first blush, one might assume that rice wouldn't grow in Canada because it's too cold, but heat wasn't the problem. It was definitely photosensitivity.
Another big aspect of growing rice in more northern climates (at least in Heilongjiang) is carefully managing the seedlings. It's actually very mechanized (and not very
permie).
I'd be interested to try some of the seed used in Vermont. I've often thought it would be fun to acquire a bit of the seed from Heilongjiang and see if it took.
Also, as an aside, this is my first post on this forum, and I couldn't help but chuckle at the way that this region thing is set up. There are a dozen shades of gray for the different regions of the US. Canada is divided into two: Ontario, and then, Canada, Alaska and Greenland. That pretty much sums up most people's perspectives, including people from Ontario.