Normally, it works this way: if the gov't owes, you have until October 15th to turn everything in; if you owe, you had better been overseas otherwise you will be penalized. The penalty is determined by how much you owe.

Go IMMEDIATELY to:
http://[url]http://www.irs.gov/[/url]

Look for the "tax forms" link.
Download the Form 4868 .pdf file.
Print it.
Guesstimate taxes and income.
If your estimate says that you owe money, include a check.
Send the form (and maybe a check) in today.
This gives you a four-month extension.

Note that this assumes your computer has Adobe Acrobat installed, and a printer on which to print stuff out.  If you ain't got no printer, you're toast.  If you ain't got no Acrobat, get thee to [url]http://www.adobe.com/[/url] and download it (assuming you have a supported operating system).  It's free for the output-only version.

It's late, sure, but better a week late than longer.  As long as they owe you, TECHNICALLY they aren't supposed to screw you over.  In reality, they might very well do so anyway.