Currently if you earn over $100,000 you are not eligible to convert your Traditional IRA to a Roth IRA. The Tax Increase Prevention and Reconciliation Act of 2005 abolished the income limit and the change will take effect as of Jan. 1,2010. The government has also included a one-time option to spread your tax payment over two years. If you convert in 2010 you could pay 50% of the tax owed in 2011 and 50% in 2012. You will need to plan ahead to decide whether it is better for you to pay the total tax bill in one year or two.
Whether you should choose to convert or not is another question. This depends on whether you would come out ahead paying the taxes at the time of conversion or when you retire.
Also as noted in a previous 2010 Traditional IRA conversion post the removal of the income limits for conversion creates a loophole that effectively removes the income limits for contributing to a Roth IRA.