NR-73 is mainly for your own benefit, so the CRA doesn't come after you later. If they get suspicious they may want you to file one. Frankly even if they do decide you're non-resident, a court can decide otherwise, at the end of the day the CRA doesn't enforce the laws, the courts do. The CRA are just a bunch of bureaucrats.
You put the departure date on the tax form for when you left, if you left in 2007 you put it on your T1 for 2007.
The advice I've given to others is that it's best to put your departure date as 31st December, otherwise you have to figure out what portion of the personal exemptions you are entitled to (plus you have to file dual-status in the US). This is the date you are claiming your tax residency moved, not necessarily the date you physically moved. You may end up paying more tax this way, but if you moved to the US during 2007 and you're claiming the tax treaty exemption it doesn't really matter from a residency standpoint, because you're saying you still live in Canada. US and Canada are okay with that.
The non-resident T1 is only used in limited situations, if you have moved your tax home to the US and you declare you have left, you don't file a tax return in Canada anymore.
Non-resident T1s are mainly for people who are residents of Canada for tax purposes but don't have residency in a specific Canadian jurisdiction, e.g. oil rig workers, soldiers stationed overseas, etc. It can come up with people who live in the US but then you're subject to State income tax in the US (which won't matter in New Hampshire as there isn't one, so yes, you can save money filing non-resident but you might be able to save more moving your tax home, providing cutting residential ties doesn't threaten your immigration status).
Read this:
http://www.cra-arc.gc.ca/E/pub/tg/p151/README.html
Steve.