If you were physically present in Canada for more than 90 days in 2007, that is a residential tie (and so is having a driver's licence) and you legally must file in Canada. You can claim dual status (i.e. only pay for the days that you were in Canada) but if you do that it's a world of complication and I would only do it if you plan on permanently residing in the US and you save significant amounts of money doing it that way (for the year 2007).
It's really complicated to file dual status because you have to calculate your personal exemptions etc. based on the number of days you were in Canada, then on your 1040 you have to do the same for the US and it's a monster headache.
What I would do is on your T1 you simply put you left on the 31st December and just file as normal (if you plan on remaining in the US permanently). You declare any US income for 2007 on your T1. You file Form 1040NR in the US and file also Form 8840 to claim the tax treaty exemption. Next year you file a 1040 like everyone else does, if you plan on residing in the US permanently.
If you really want to do dual status then you have to do three tax returns, a T1 for the period you were in Canada with the date that you left, a 1040NR for that period in the US and a 1040 for the period after you moved to the US.
Read the instructions in depth for both the 1040NR and 1040 for dual status filing. Also the instructions for the T1 for the same reason and how to declare US income and working out the exchange rate.
Read this:
http://www.cra-arc.gc.ca/E/pub/tg/p151/README.html
Note also that if you entered the US as a permanent resident you HAVE to file dual status for 2007.
If you plan on remaining in a non-immigrant category and you plan on coming back to Canada at some point, you just file as it describes in that CRA publication above, i.e. T1 as normal (well, not completely normal, you have to declare US income) and in the US you file a 1040NR and an 8840.
I can't see why you can't file together with your husband, but you must both claim the tax treaty exemption in the US and both file jointly on a T1 in Canada. You can't have a situation where one of you has a tax home in Canada and the other in the US.
Be careful also with your bank, you have to tell them what your status is. I.e. if your tax home is Canada or the US. If it's the US, by law they have to withhold Part XIII tax, i.e. 10% of your bank interest. If you fail to notify them, the CRA can take that as a "residential tie". Or worse yet, you end up in the reverse situation, they withhold the tax because you've given them a US address, but actually your tax home is still Canada, so then you have to claim the Part XIII tax back on Form NR7-R.
Steve.