Residency in Canada for tax purposes is only 90 days actually, so yes, you were resident for tax purposes in the 2007 tax year.
You really need to talk to an accountant, but I think the trick here is to claim a closer association to Canada for the whole year, so you file a 1040NR and an 8840 for 2007 and you pay tax to Revenue Canada. For 2008 you do it in reverse, non-resident in Canada and resident in the US.
Otherwise you're going to be paying tax to two different places for 2007, which could get complex. But it depends on your specific circumstances. If you've changed jobs, have a T-4 and a W-2 (i.e. paid witholding in the US), self-employed, etc., you need an accountant or thoroughly read up on it on the Revenue Canada site and
www.irs.gov
For example it might be simpler to do two tax returns, based on your T-4 for Canada on your T-1 and your W-2 on your 1040NR.
The instructions for the 1040NR are pretty comprehensive, they explain the tax treaty provisions you can claim.
You need to inform your bank in Canada that you are no longer resident in Canada and get your address changed to a US address - if you don't they will file a T-5 with Revenue Canada and they will take that as proof of residency in Canada. As soon as you become non-resident, your RRSP and bank interest will be subject to Part VIII tax, which means 10% tax on all income from those sources.
Steve.