Refer to Ed Slott at www.irahelp.com - he is the foremost expert on the every changing rules surrounding IRA's and related tax laws
Yes, you do need to pay in the same year. When the regulation removing the cap on income for Roth IRA conversion first went into effect in 2010, there was a one-time opportunity to defer the tax, half to 2011 and half to 2012 but that is no longer in effect.
As the other answers point out, you are required to pay the taxes on the conversion for the tax year the conversion was completed. This is a real good reason to meet with an advisor and review your current tax situation - because you don't have to convert ALL IRA money in one tax year but can spread the conversions over several years and pay less tax by knowing where your taxable income is realtive to the tax brackets in any given year. For example, if your taxable income is $10,000 lower than the top of the 25% bracket and your IRA balance is $20,000, you can convert (up to) half of the IRA this year at a tax cost of 25% of hte converted amount instead of converting the entire balance and getting bumped into the next bracket and paying 28% on half of the converted amount. You can decide each year how much tax you are willing to pay to do the conversion. Being aware of the conversion rules and where your taxable income is relative to the tax brackets is key. Good luck!
