When To File Form 4868

File Form 4868 by April 18, 2011. Fiscal year taxpayers, file Form 4868 by the regular due date of the return. Taxpayers who are out of the country. If, on the regular due date of your return, you are out of the country and a U.S. citizen or resident, you are allowed 2 extra months to file your return and pay any amount due without requesting an irs extension. For a calendar year return, this is June 15, 2011. File this form and be sure to check the box on line 8 if you need an additional 4 months to file your return.

If you are out of the country and a U.S. citizen or resident, you may qualify for special tax treatment if you meet the foreign residence or physical presence tests. If you do not expect to meet either of those tests by the due date of your return, request an extension to a date after you expect to qualify using Form 2350, Application for Extension of Time To File U.S. Income Tax Return.
You are out of the country if:

• You live outside the United States and Puerto Rico and your main place of work is outside the United States and Puerto Rico, or
• You are in military or naval service outside the United States and Puerto Rico.

If you qualify as being out of the country, you will still be eligible to file tax extension even if you are physically present in the United States or Puerto Rico on the regular due date of the return.
For more information on extensions for taxpayers out of the country, see Pub. 54, Tax Guide for U.S. Citizens and Resident Aliens Abroad.

Form 1040NR or 1040NR-EZ filers. If you cannot file your return by the due date, you should file Form 4868. You must file Form 4868 by the regular due date of the return.

Source:  http://www.irs.gov/pub/irs-pdf/f4868.pdf