January 15th is the last day you can pay estimated taxes for 2009 without worrying about the IRS’s 4% interest penalty. For most people, you need to have paid 90% of what you owe for 2009 or have a good reason why you didn’t (e.g. casualty, retirement). Kiplinger notes that even if you can’t pay the full amount, pay whatever you can by January 15th to reduce the amount that’s penalized.
“Last Chance to Avoid Underpayment Penalty” [Kiplinger]