Starting in 2014 for U.S. citizens if you don't have the "minimum essential coverage" you have to pay a penalty to the IRS.
My understanding is that U.S. citizens residing abroad (as described below) are considered to have the "minimum essential coverage" (even if they don't have any medical insurance).
The Patient Protection and Affordable Care Act. see page 131
INDIVIDUALS RESIDING OUTSIDE UNITED STATES OR RESIDENTS OF TERRITORIES.—Any applicable individual shall be
treated as having minimum essential coverage for any month—
‘‘(A) if such month occurs during any period described in
subparagraph (A) or (B) of section 911(d)(1) which is applicable to the individual
http://www.gpo.gov/fdsys/pkg/BILLS-111hr3590enr/pdf/BILLS-111hr3590enr.pdfUS Code -
Section 911: Citizens or residents of the United States living abroad
(d) Definitions and special rules
For purposes of this section -
(1) Qualified individual
The term "qualified individual" means an individual whose tax home is in a foreign country and who is -
(A) a citizen of the United States and establishes to the satisfaction of the Secretary that he has been a bona fide resident of a foreign country or countries for an uninterrupted period which includes an entire taxable year, or
(B) a citizen or resident of the United States and who, during any period of 12 consecutive months, is present in a foreign country or countries during at least 330 full days in such period.
http://codes.lp.findlaw.com/uscode/26/A/1/N/III/B/911Basically what the above says is that you cannot be in the U.S. for more than 35 days during any 12 consecutive month period in order for the IRS to consider you as residing abroad for that 12 month period. (This is the same rule to qualify for the Foreign Earned Income Exclusion.)