Mmm, under the tax treaty if you work in the US for more than 183 days a year OR receive more than $10,000 in compensation while working in the US you have to pay
taxes in the US which obviously implies that you have to file a US tax return. However the obligation here is largely on your employer, because if they have employees who spend more than 183 days in the US or earn more than $10,000 while there, they're supposed to register as a US employer, do US payroll tax withholding, and issue you a W-2 for the portion of your salary paid to you while you were in the US.
The 2008 tax treaty also requires proportional payment of taxes although I haven't seen any clear guidance on how that works yet, essentially though if your employer has a "permanent establishment" in the US, which means an office manned more than 183 days a year, or where more than 50% of the company's work is performed, then they must be registered with the IRS and do payroll withholding for all the workers who work there while they are in the US.
So to cut a long story short the obligation is on your employer to comply with US tax laws. If they don't issue you a W-2 or do US payroll withholding, filing a 1040NR would be rather messy because your income doesn't appear to be US-source.
I think your first port of call would be to talk to the company you work for and see whether they're aware of all this.
IRS publications 515 and 597 contain more information for employers, although it is significantly out-of-date now due to the new tax treaty provisions.
There are specific exemptions for people who work in the transport industry, if that's what you do.
Steve.