Perhaps I am getting ahead of myself -- I have been assuming that we will not have a federal estate tax for 2010. It's possible that Congress might get its act together and actually pass an estate tax bill in 2010 that applies for both 2010 and the future. This is what I've always thought would happen (naive me) -- maybe permanently setting the exemption at $3.5 million.
But this raises the question of what happens with individuals who die in 2010 before the new law, reinstating the estate tax, passes. Could such a law be made retroactive?
Probably. The Supreme Court previously stated (in Carlton v. United States, 512 U.S. 24 (1994)) that a retroactive law is valid under the Constitution if (1) the government shows that the statute has a rational legislative purpose and is not arbitrary and irrational; and (2) the period of retroactivity is "modest." (In Carlton, the "modest" period of retroactivity was 14 months.)
That being said, there is some caselaw indicating that the result might be different if the Supreme Court views this law (estate tax reboot? estate tax 2.0?) as a "wholly new" tax or as simply fixing something in an existing tax (the Carlton case mentioned above involved closing an estate tax loophole).
You may want to take a look at this article on Gideon Alpert's excellent Gay Couples Law Blog for a bit more information on this topic.