Also of use:
http://www.grasmick.com/nafta.htm
According to this site it seems like it'd be fairly easy for Canadians to go and work for a few months in the US. I'm not a lawyer, so not 100% on any this but it seems fairly clear. I'm sure YC has real lawyers who will work out any details. This is just mostly to put my own mind at ease about trouble getting in. And it looks like it won't be much trouble at all.