When the Senate allowed the estate tax to lapse at the end of last year, it encouraged wealthy people near death's door to stay alive until Jan. 1 so they could spare their heirs a 45% tax hit.

Now the situation has reversed: If Congress doesn't change the law soon—and many experts think it won't—the estate tax will come roaring back in 2011.

Not only will the top rate jump to 55%, but the exemption will shrink from $3.5 million per individual in 2009 to just $1 million in 2011, potentially affecting eight times as many taxpayers.

The math is ugly: On a $5 million estate, the tax consequence of dying a minute after midnight on Jan. 1, 2011 rather than two minutes earlier could be more than $2 million; on a $15 million estate, the difference could be about $8 million.