ianal but my understanding is that americans do have to pay us income tax on worldwide income, but they can deduct taxes paid to a local government. in other words, if you would have had to pay $100 in taxes to the us, but paid $75 to a local government, then you would only pay $25 to the us. if your taxes in the foreign country exceed the amount you would have had to pay in the us, then you don't owe the us anything.
Between the Foreign Tax Credit and the Foreign Earned Income Exclusion... You'd need a fair amount of US income or very low taxes in your new home country to generate a US tax burden. But. You still have to file.
