Which gives you an exemption for $100,800 (2015) and potentially some housing costs. Worldwide income above that is still taxed even if you never step foot in the U.S.
I think form 2555 applies if you're actually working overseas (as in employment). If you have a trading account at TD Ameritrade, for example, and rent an apartment overseas where you place all of your trades, you would not qualify for that exemption. Otherwise, traders would simply take their laptops and operate from a beach in Tahiti, while paying zero tax on the first $100k of trading gains.

From the instructions on Form 2555:
"Foreign earned income for this purpose
means wages, salaries, professional fees,
and other compensation received for
personal services you performed in a
foreign country during the period for which
you meet the tax home test and either the
bona fide residence test or the physical
presence test."
https://www.irs.gov/pub/irs-pdf/i2555.pdf