He is using the transfer taxunifiedcredit, which has a lifetime limit.
FORBES: Mitt Romney Tax Clarifications
2.
Every individual can pass property of a value up to the up to the unifiedcredit free of Federal estate tax.
FORBES: Estate Planning in the Current Environment
3.
If all works as planned, at the death of the survivor of them the couple passes property of a value at least equal to twice the unifiedcredit to the next generation free of Federal estate tax.
FORBES: Estate Planning in the Current Environment