Under the law signed last December, for 2010 through 2012, taxable estates exceeding 5 million dollars are subject to the US Estate Tax. There is a "portability" feature in this law, permitting couples to transfer the unused portion of the first to die to the estate of the surviving spouse.
The 5 million exemption expires Dec. 31, 2012. Then. it's anyone's guess. Elections have consequences.

A point of hairsplitting clarification JR.
There is no federal "Inheritance" tax. The proper federal term is the "Estate" tax. Some states, like mine (NJ) have an inheritance tax. Some states also, like mine, have both an "estate" and an "inheritance" tax. Here in NJ, it's two different taxes with two different histories and applications.