Thanks for responses so far. I am in fact very much interested in Canada specifically. Based on Bosco's response sounds like Roth money would then indeed be taxed twice... !? :-(
But I realize this is a guess.. hoping others will contribute too.
In general, you need to realize that if you move to Canada as a US citizen you will pay more taxes.
The issue is not that taxes are all that much higher in Canada. If you consider your Canadian income tax to include what you would be paying in the states for health insurance premiums, they are fairly similar.
The problem is the tax treaty. The US recently changed how they interpret it. As a rough/crude example, Let's assume the amount of money on which you are not "double-taxed" under the treaty is $85,000 (it may be just a bit higher now).
Let's say your income is $105,000.
You shouldn't owe much US tax, right? Only tax on $20,000 in income? Not so fast. You are taxed on $20,000, but you are taxed on dollar $85,001 to dollar $120,000.
You will be filing dual tax returns, and likely remitting money to both governments.
Canada is a wonderful place to live, but the US will not release its talons from you easily--you would have to renounce your US citizenship and wait 10 years. A fairly drastic move.
If, like me, much of your income is US-sourced, then you will pay US income tax on that income, then write a cheque to the Canadian government for your tax bill in Canada, while receiving a credit on the tax you paid in the US. Renouncing would only make things worse for you. Also, envision yourself trying to tell a US customs agent why you chose NOT to be American. They might not fully understand....
There is a firm called Serbinski (google it) that has a lot of info on cross-border tax issues, and a forum.
Here's a link to the forum
Serbinski Accounting Firms :: View Forum - Canada / United States Tax & Accounting
In general I have simplified a complex topic.
Disclaimer: I am not an accountant, tax lawyer, or anybody knowledgeable--just a poor schmuck trying to get by. And yes, my US tax credit (which I think is legit) is being audited by Revenue Canada. And if you thought the IRS is bad--you ain't seen nothing until you deal with these guys. Plus, the Canadian system is even more byzantine and arcane, if you can believe that. Having lived and worked on both sides of the border, I can say that the system is not very well set up for those of us with one foot on either side.