If you're directly employed by them they have to do Canadian payroll
taxes (in which case it's probably going to be easier to register as self-employed and become a subcontractor). It's one or the other because you must be paying into CPP, it's legally required on employment income.
How you do this depends on a number of factors.
If none of the work is performed in the US and the amount you receive is less than $100,000 annually then I don't think there's anything you need to file in the US. If you receive over $100,000 you have to file a completed 8833 with the IRS to expressly claim tax treaty provisions (this is a bit vague - the IRS never seem to have a straight answer on what the limit is because it depends on the source).
If you ever physically do any of the work in the US then you need to file a completed Form 8233 with them in order to exempt your employer from non-resident alien withholding. They also have to file 1042-S and 1042, although the IRS will usually give them a waiver for this if all their foreign subcontractors are Canadian because there is no withholding. In your situation they should be giving you a 1042-S each time they pay you from the sounds of it.
For individuals I don't think you do need to file a US tax return in this situation as none of the work was performed in the US so there is no US tax, plus you're exempt from NRA withholding, so there's nothing to claim back. The IRS can sometimes get snotty and ask for one though, it's called a "protective" return, i.e. a CYA tax return and you make them aware there is no tax. You'd need to phone them and ask, usually it comes up if the amounts involved are quite large and the IRS wants to know where all the money is going to ensure there is no fraud involved.
Corporations usually do have to file an 8833 at least and a protective 1120-F return because the amounts involved are usually large enough.
Anyway this should give you a rough idea, like I said, call the IRS so you can explain your specific situation to them and see what they say: 215-516-2000.
Steve.