There was a time when irrevocable bypass trusts were highly favored by estate planning attorneys as one of the best estate planning methods for married couples. It worked like this: one spouse would fund the trust with an amount that was just under the estate tax exemption. At the time that the funding spouse passed away, funds in the trust were available for the heirs, and the balance of the estate was inherited by the surviving spouse.
Consequently, this approach lowered the size of the surviving spouse's eventual estate and lessened the estate tax burden for the married couple. However, as Kiplinger's Retirement Report points out in "Old Trusts Create Tax Issues for Heirs," estate tax laws have changed significantly since the time when many of these trusts were created.
The estate tax exemption is far higher than it used to be, and spousal portability now allows a married couple to double its estate tax exemption.