Regardless of any extensions you receive or apply for as an expat, any taxes that are due for 2011 must be paid by 4/16/12 in order to avoid interest and penalty assessments for paying any taxes due at a later date. It is important (if you return is not completed yet) to file your expat extension by 6/15/12 since that will stop the IRS of assessing the largest penalty which is 5% (of the tax due with the return) per month up to a maximum of 25% (of the tax due with the return) for filing your return late.
Your FBAR form TDF 90-22.1 (filed to report foreign bank and financial accounts) must be filed no later than June 30, 2012 for the 2011 Calendar year. That form is filed separately from your tax return and cannot be extended beyond that date.
Most of the other special forms filed to report your offshore activities such as 8938, 5471, 3520, 8865, etc., due on the extended due date of your personal tax return. The one exception is Form 3520A( filed for foreign trusts and Mexican Fideicomisos) is due 3/15/12 for 2011, but can be extended with Form 7004 to September 15, 2012.