The extension is for filing. There is no extension for payments. This is directly from the IRS website:
IRS Offers Reminders as Tax Filing Deadline Nears
IR-2009-40, April 10, 2009
WASHINGTON ó The Internal Revenue Service offers last minute reminders to taxpayers who have not yet filed a tax return, paid what they owe or requested an extension of time to file as the April 15 tax filing and payment deadline approaches.
File and Pay on Time
Taxpayers who owe taxes and donít file their tax return by the deadline may face interest on the unpaid taxes and a failure-to-file penalty. Interest and penalties add to the total amount a taxpayer owes. Filing by the deadline allows taxpayers to avoid the failure-to-file penalty, even if they canít pay all or some of their taxes by the deadline. Taxpayers who canít file their return by the deadline can request an extension of time to file. However, an extension of time to file is not an extension of time to pay.
Taxpayers who file on time but donít pay all or some of their taxes by the deadline could face interest on the unpaid amount and a failure-to-pay penalty. Taxpayers who canít pay the full amount should pay as much as they can by the deadline to minimize any interest and penalties due. In addition, taxpayers may take advantage of a variety of electronic and other payment options, such as using charge or debit cards to pay their taxes, to make it easier.
Taxpayers may also pay any taxes owed by check made out to the ďUnited States TreasuryĒ using Form 1040-V, Payment Voucher, which must be included along with the payment and tax return. Taxpayers who have already submitted their tax return, but still need to pay all or some of their taxes, may mail the check to the IRS with Form 1040-V.