Virtually all estate plans made before 2010 assumed -- as all the experts predicted -- that Congress would extend the federal estate tax beyond its scheduled expiration of January 1, 2010. But Congress let the tax lapse. Senators are now talking about bringing it back and making it retroactive to January 1, but until they act no one can be sure what law will be in effect for deaths in 2010.

Many people trying to avoid the estate tax set up trusts with "formula clauses," which divvied up property based on the tax consequences. For example, in a trust for their surviving spouse, somewhat might have left an amount "equal to the amount that can pass free of federal estate tax at my death," expecting that amount to be $3.5 million (as it would have been in 2009). But now, with no tax, those plans no longer work as their makers intended.

Several states have set out to make sure that people's wishes aren't thwarted by the unexpected change in tax law. Indiana is the first state to actually pass a bill (Senate Bill 65) that changes the way wills and trusts are interpreted. It instructs executors and trustees who encounter a formula clause to apply the federal estate tax laws that were in effect on December 31, 2009 for the purposes of creating two separate trusts. If Congress brings the federal estate tax back in 2010, the state law will cease to have any effect.

Florida, Maryland, Nebraska, South Dakota, Tennessee, Virginia, and Washington are also considering laws to address the problem.