As I understand it, 90% of the previous tax years (2008) tax liability needs to be paid in by Jan 15th, 2010 or had to be withheld from their 2009 W-2, 1099-R etc.. or a combo of the 2 Correct?

A client calls yesterday and withdrew money from their annuities in 2009. No income tax was withheld nor mailed in and they believe they will owe $10K of income taxes on that transaction. I myself have not yet look at it. In 2008 this clients computed Fed income tax was $10500 and had $12K withheld from 1099-R's therefore received a refund but did not increase their Fed withholding in 2009. Assuming the $10k is the correct amount, does my client need to mail a check to the IRS asap to minimize any interest and/or penalties? OR can my client wait and pay the additional Fed income tax on 4/15/10 (via credit card) and not be charged any interest and/or penalties?