There is a July 2008 change to tax treaties.
The changes include:
Since you have been living and working in the U.S. you are treated as a resident.
When you retire and leave the U.S. with your money rolled into an IRA account, provided the laws do not change by then, the taxing is directed by treaty.
There are provisions also for transfering 401(k) to an IRA and subsequent transfer of an IRA to an RRSP.
By treaty, if your Roth IRA distributions would not be taxable in the United States, they would also not be taxable in Canada. Roth IRA distributions, are in fact, tax free, because they were established with after tax contributions. There for if the money is in a ROTH IRA, there will be no tax on distributions.
NOTE: except for rollovers, if you make contributions to the Roth IRA while resident in Canada, your Roth IRA will be bifurcated, with the portion contributed while you were in the U.S. being received as distributions without tax, and the portion of distributions related contributions while being resident in Canada as being taxed.
Under the new rules, you would be taxed on your regular IRA distributions from the U.S. IRA account, and on the canadian side, since this is retirment benefits, you would add the income to your Canadian income tax return, and take a tax credit for taxes paid to the U.S.