The penalty for an individual that does not make estimated tax payments during the year and pays the liability on or before the due date of the tax return is currently 3% annual rate for the first quarter of 2011.
Historically that rate has been between 4-8% from 2004 through 2010.
For a chart of rates by quarter see http://www.fairmark.com/reference/index.htm
(near the bottom of the page)
Under the Internal Revenue Code, the rate of interest is determined on a quarterly basis. For taxpayers other than corporations, the overpayment and underpayment rate is the federal short-term rate plus 3 percentage points.
Since the federal short term rate is not likely to increase in 2011 much, if any, the underpayment rate will likely stay at 3% for most or all of 2011.
Please ask if you need clarification.