Last year, Congress allowed anyone to convert a 401(k) or IRA account to a Roth IRA, lifting the restriction that barred the conversion to those earning $100,000 a year or less.

Congress also allowed taxpayers to give up to $5 million to grandchildren, up from $3.5 million, without incurring the generation-skipping tax. That exemption was supposed to drop to $1 million if Congress hadn’t acted and is now scheduled to drop in 2013.

If your clients have money in retirement accounts they won’t need to live on, they can put that money into a Roth IRA and avoid paying taxes on any gains. Now, grandkids can inherit up to $5 million tax-free.

A grandchild will benefit more than a child because of a longer life expectancy. Roth inheritors can spread distributions out over a lifetime. This means a one-year old who earned an average annual return of 8% on that $5 million could get a lifetime income of $408 million, completely free of taxes, IRA expert Ed Slott calculates.

Double that amount, if two grandparents give $5 million each.

Because Roth holders don’t need to take distributions after turning age 70 and a half, the Roth IRA is widely considered to be the best way to save assets for heirs. Taxpayers do have to pay income taxes on the assets moved over from another kind of retirement account. At the highest rate of 35%, the tax bill for converting $5 million is about $1.7 million.

But Congress may close this estate-planning deal before 2013. For now, Slott doesn’t expect retroactive changes affecting people who took advantage of the tax break while it lasted.

“They’re not going to change it backwards. They have enough trouble going forward,” he said . "This is probably as good as it’ll get."

Clients should do the conversion as soon as possible to get their money growing tax-free. Taxpayers have up until Oct. 15 of the following year to undo the conversion.

Slott’s comments drew him into the political fray when he was quoted by Karen Hube, a columnist for the Fiscal Times, in a story, “The Tax Law That Can Make Your Kids Super Rich,” which was then picked up by the Washington Post.

He first mentioned the tax break in his Financial Planning column in March.