By "haven't formally told the Canadian Govt. that I've left" I take it you still file
taxes in Canada? I.e. you file a 1040NR in the US rather than a 1040 every year and claim a foreign tax credit in Canada?
If you're filing a 1040 (i.e. your tax home is the US) you're treated as a resident alien by the IRS, and if you are in the process of getting LPR status you will basically be screwed forever if you give that up and move your tax home back to Canada, if you want at some point to move back to the US. You could still file taxes in the US and claim to be a non-resident in Canada, but it's hard to pull that off if you're a Canadian citizen and you have only temporary status in the US, the CRA won't believe you.
If you're moving permanently back to Canada then it's not as big an issue, and it's a pretty trivial issue if you currently file 1040NR and you abandon your LPR application.
How it works for non-resident employees is that you file a 1040NR and an 8833 every year to report your income to the IRS and claim a tax treaty position. In Canada you claim a foreign tax credit for the US income tax. (You benefit from the social security tax under the totalization agreement.)
But if you're currently filing 1040 you need to move your tax home back to Canada first (which is fairly complicated).
The tax situation for the employer can be quite complicated, IRS publication 515 explains how to do it. If none of the work is physically done in the US then it isn't too difficult, have a read of pages 22-26 of that publication.
Generally though it's simpler to be self-employed in Canada and be an independent contractor, you file Form 8233 with them every year to claim a tax treaty provision so they don't have to do non-resident withholding, you send them invoices, they pay you, you declare it every year on your tax returns, i.e. 1040NR/8833 and your T1. They may have to file Form 1042, IRS gives me vague answers on that one, I think they like all employers to file them if they have non-resident employees even if it's a nil return but if there is no actual withholding required the employer doesn't have to.
Forms 8233 and 8833 are pretty difficult to fill out and the tax treaty changed this year so all the instructions are out-of-date (if in fact you could understand them - I've never been able to). Really you need to talk direct to the IRS on how to fill out those forms, or a very good accountant who is really up-to-date on it. Form 8233 will probably be changed to reflect the new definitions in the treaty, but I haven't seen the new one yet.
Have a read of IRS publication 519 also.
You can visit the US as a regular B-1 business visitor to have meetings with them, that isn't a big deal.
Steve.