The Affordable Care Act (Obamacare) aims to fund the provision of healthcare to all US citizens partly via a 3.8% surtax on net investment income.
However it will only apply to taxpayers whose modified adjusted gross income exceeds certain thresholds ($200k for singles, $250k for married filing jointly, $125k for married filing separately).
The foreign income exclusion and the foreign housing deduction are not allowable in calculating the modified adjusted gross income.
The 3.8% surtax is therefore applied to the lower of
* net investment income, or
* the surplus MAGI over the relevant threshhold
In practice most taxpayers will only pay on their net investment income.