The great American death watch created when the estate tax was eliminated at the beginning of this year has taken on a new twist.

As the hoopla surrounding this week's tax compromise weaves it way through Congress, there s one group that tax pros say may be left in the lurch when the dust settles: people who inherited assets in 2010. Sure, plenty of jokes have been made about pulling the plug on an ailing rich mother before the clock strikes midnight on Dec. 31 to avoid paying the federal estate tax. But that may be the worst possible thing to do financially (never mind, ethically) for the heirs, who may get stuck paying exorbitant capital-gains taxes once they sell the assets. Anyone who inherits in 2010 will have a big surprise when they go to sell, says Linda Gould, tax attorney for the National Association of Realtors.

Here s why and it s complicated. For most of the history of the estate tax, a provision called step-up in basis allowed heirs to value assets the home, the stocks, the cars at current fair market value, instead of the original purchase price. If the total value of assets exceeded a certain threshold ($3.5 million, in 2009), estate taxes had to be paid, but when an heir sold an asset, he might earn much lower taxable capital gains or even none.

When the estate tax was eliminated for 2010 as part of the 2001 tax-cut legislation, with it went the step-up rule. Executors of an estate are still able to assign stepped-up status to $1.3 million of assets in the estate and up to $3 million for assets left to a surviving spouse, but 2010 heirs would still, at the end of the day, have to pay taxes on the gains based off the original price of the asset.

Here s an example. Say you inherited your mother s home this year. She bought it for $200,000 and today it s worth $4 million. If you had inherited it last year, you would ve had to pay estate taxes on half a million (the value of the house, minus the estate-tax exclusion). But since you inherited it this year, you pay no estate tax and instead get taxed on a whopping $2.5 million gain (the $3.8 million gain on the house, minus your $1.3 million exclusion). The irony, of course, is that some people who never would ve owed estate taxes now might take a capital-gains hit. See why you don t want to kill off mom this year?

The estate tax regime is a primary concern for tax staffers on Capitol Hill. As the current tax deal stands, it s still unclear whether or how Congress will hammer out a semi-reprieve for 2010 heirs. If history repeats itself, things could get ugly. Congress previously eliminated the step-up in basis in 1976 as part of a big estate tax overhaul. Things got so complicated and difficult for the IRS to enforce that the Carter administration in 1980 retroactivity repealed that move. So when it was put in the 2001 tax rules, people were appalled. We couldn t believe that anyone would do something so irresponsible, Gould says.

Financial planners and tax experts say that the fact that the details of how to handle 2010 estates haven t been ironed out is, simply put, ridiculous, given that the government has had almost a decade to figure out how to deal with the one-year repeal. This is a remarkable failure to govern, says Benjamin Harris, a tax expert at the Brookings Institution. Congress hasn t made a bigger error. What s more, the inability to hash out repeal logistics means everyone is in a holding pattern; several wealth managers say the IRS hasn t released any guidelines on how 2010 heirs should file.

Advisors who work with wealthier clients are bracing for a myriad of scenarios. They re scrambling to collect records to prove the value of dad s ski chalet should Congress not change a thing. They re ordering up appraisals, should a last-minute addendum come through. And they re priming for litigation, which is bound to arise at some point. As for people stuck in the 2010 inheritance predicament, the first thing they should do is talk to a tax accountant. Beyond that, as Wilmington Trust managing director Carol Kroch put it, we re all waiting.