Under current law, surviving spouse may under some circumstances use the Deceased Spouse’s Unused Exclusion Amount (DSUEA) in order to avoid estate tax, potentially doubling her lifetime exclusion from $5,120,000 to $10,240,000. There are several requirements for this “portability” and one of them is filing a timely election on an estate tax return (706).
IRS has extended the time for filing this election. Notice 2012-21 here.