The substantial presence test is not something that you ever really need to worry about, it's a test of last resort, what's more important is where your principal residence is, where you hold a DL etc. That test only comes into it if it's still unclear. The question is where you substantially have ties.
You clearly are a non-resident alien if your wife still lives in Canada and you still have your principal residence there.
What you do in this situation is file a T1 as normal next year, and you claim a foreign tax credit (which is explained in the T1 instructions) using the information your employer will give you on your W-2 (which is the equivalent of a T4).
In the US you file a 1040NR together with an 8840 (and your W-2).
On your W-4 you must state that you are a non-resident alien and your employer must do NRA withholding, which is done at a slightly higher rate than for residents.
You do effectively pay more tax filing as an NRA, because you end up paying Canadian income tax + US social security tax after you claim the foreign tax credit in Canada, and US social security
taxes are higher than CPP (unless you're in a low income bracket).
However in your case there clearly is no other way to do it as you clearly have residential ties to Canada. You can't file as a US resident until you sever your ties to Canada.
More info in IRS publication 519 and pages 22-26 of IRS publication 515.
If you want to move your tax home to the US it's better if you can do it on January 1st because it will save you having to file dual-status in 2010. It is really not a good idea to move your tax home to the US if you only plan on being in the US temporarily (i.e. a year or two). The paperwork to move it there and then move it back is pretty intimidating.
If you read IRS publication 519 it will give you an idea, but effectively, if you move your tax home to the US in the middle of next year, you will have to file in 2010:
A 1040 return (dual-status);
A 1040NR return (dual-status);
A T1 return (and maybe a dual-status non-resident T1);
Probably an 8891 to declare RRSPs if you have any;
A FinCEN form declaring any foreign assets over $10,000;
And possibly other bits of paperwork, 8840, 8833, 1116, etc.
To do it in reverse is even more complicated, you have to file those three tax returns again plus a 1040-C and a variety of other bits of paper.
Like I said, if you decide to do it, you can save yourself a lot of grief if you can move your tax home on January 1st because then you only have to file one tax return the following year rather than three.
If you screw it up when you leave and you move to Canada or the
UK there is a good chance the IRS will come after you, it happened to one of my relatives and the IRS has offices in London and Ottawa.
From a paperwork standpoint it is easier to continue to file as a non-resident if you ever plan on moving back to Canada, but hardly anyone does who stays for any length of time as you end up paying more tax.
In addition the IRS doesn't like people filing as an NRA for more than five consecutive years if they have a US address on their 1040NR.
Steve.