Shalab,
As long as your payments cover 100% of your tax liability from the prior year, there will be no underpayment penalty. You can do this even if you know the coming year's tax bill will be higher and you'll just have to make up the difference come filing time. Realize that if your AGI was over $150,000 (married, filing jointly) the previous year, then the safe harbor is 110% of last year's tax due.
If you plan on splitting the conversion income from a Roth conversion in 2010 on your 2011 and 2012 tax returns, your 2011 estimates would be based off of your 2010 tax return using the rules above.