If you marry a US citizen and do AOS and become an LPR, by law you MUST file as a resident of the US and you must declare all your worldwide income to the US Govt. and pay US
taxes on it (if there are any). You must also file jointly with your US spouse.
If you have any Canadian-source income you file in Canada using the non-resident T1 (assuming you have reportable income, read the instructions). You MUST sever all residential ties to Canada pretty much, DL, OHIP, etc., otherwise you may end up being considered a resident of both places in which case you would end up getting taxed twice.
In the first year that you file as a resident of the US, you must file as "dual-status", i.e. non-resident for the first part of the year and resident for the second part of the year. You may also need to do this in Canada, i.e. resident for the first part of the year and non-resident for the second part of the year. This is why it's always best to move your tax home on January 1st if you can pull it off, because it saves you a lot of paperwork. If you move tax homes mid-year then you have to pro-rate your personal exemptions based on the number of days you lived in either country.
There is a "departure tax", essentially how this works is that it is a capital gains tax of 25% on your assets, excluding your principal residence and $50,000 of assets. So for the average Joe you're unlikely to be hit with it. If you have more than one home or you have substantial investments outside of a tax shelter (i.e. excluding RRSPs and the like) then you may be hit with it. You are deemed to have disposed of your assets on the date of departure at FMV, as determined by the CRA.
Under the 2008 tax treaty, distributions inside of RRSPs are no longer subject to US income tax, so when you file 1040 all you have to do is declare them to the IRS on Form 8891. You may also need to file the FinCEN form for foreign assets, which you must file for assets outside the US worth more than $10,000 - this is not an IRS form, it is a Treasury Dept. form.
If you have Canadian-source income subject to tax then you can claim a foreign tax credit in the US on Form 1116 so you don't get taxed twice after you've filed your non-resident T1.
Have a read of IRS Publication 519 which explains how to do a dual-status return, once you've moved your tax home though I'm afraid you've got to start hunting through the more obscure instructions in the regular 1040 package as obviously most US taxpayers don't have foreign-source income from things like CPP.
This contains more info on the departure tax:
http://www.cra-arc.gc.ca/tx/nnrsdnts/nd ... n-eng.htmlFinCEN form:
http://www.fincen.gov/forms/files/f9022-1_fbar.pdf
Steve.