I am 51 years old and withdrew $130,000 from my 401K in 2011. At the time of withdrawal, 20% was held back for taxes by my 401K plan provider.

How much additional tax will I have to pay at tax time?

It depends on your other income and deductions or credits. That $130K is included as ordinary income and so it may put you into a higher tax bracket than 20% - there is no way for us to tell. But in addition to income tax you also have to pay the 10% early witjhdrawal penalty, which is $13,000. And depending on where you live you may have state/local income tax pay on the $130,000 as well - if we assume a 6% state income tax rate that would be $7800.