The IRS charges interest on any tax that remains unpaid by the April due date. This means that if you owe tax and have not paid the full amount due by the April deadline interest will accrue on any unpaid amount.
This applies whether or not you have taken an extension of the time to file, or whether the IRS has agreed to waive any of the penalties usually added.
Note that interest is also charged on the penalties themselves, again from the official due date of the return.
The current interest rate on unpaid tax, including penalties, is 3 percent. Federal tax underpayment interest rates are adjusted quarterly and can vary quarter by quarter and, of course, year after year. In 2007, for instance, the interst rate remained fixed at 8 percent throughout the year.