There are no underpayment rules unique to Roth conversions, 2010 or otherwise. That means the usual safe harbor measures remain in place, ie 100% of prior year tax liability (110% if AGI over 150,000) or 90% of the current year tax liability. If estimates are used, they must be paid in equal installments or up front, not back loaded unless you want to do the miserable annualized income installment forms. Few people want to deal with that form.

For 2010 conversions, the default income reporting is 50% for 2011 and 50% for 2012, nothing in 2010. That means that if your 2011 estimates equaled your 2010 tax liability there would be no underpayment penalty for 2011. No penalty, but you would owe extra taxes in April, 2012. For 2012, estimates would have to be increased to reflect the higher 2011 taxes, and in 2013 you would probably change over to the 90% of current year measure to avoid overpaying estimates.

But for regular conversion years where you report the full conversion in that same year, all you have to do is pay in the prior year tax liability to prevent a penalty. For 2010 conversions you can opt out of the deferral and report the entire amount in 2010 if you wish, eg you find out that 2011 tax rates went up. In that case, the 2010 conversion would be like any other year in paying estimates.

If you have withholding sources, you can combine them with estimates or even cover smaller conversions with higher withholding. Unlike estimates which are credited when paid, withholding is deemed to be paid equally over the entire year, meaning that you can make up a prior deficiency with a large one time withholding at year end.

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This is a zombie Post from a > 12 year-old thread.
Not to mention, the prior post answers your questions.

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