Bloomberg has a nice, detailed article, here.
Good news though: If you are overseas, you have until June 15, 2013 to file; however, you have only until today to pay. Otherwise, you are subject to that 3% a year underpayment penalty.
If you want the Foreign Earned Income Exclusion, you must file by June 15 to get that.
If you want to extend the June 15th deadline, you need to file Form 4868.
Most importantly, the newest wrinkle: If you expect come back to America and be covered by the Affordable Care Act (“Obamacare”), if you use a Health Insurance Exchange, you better have a copy of your 2012 tax return. Foreign Earned Income is an “addback” to determine modified adjusted gross income for Obamacare premium credits. Nonfiling is bad enough, but if you also hide that income and obtain Obamacare credits, you’ve just made things worse.
Best thing: FILE.