Is the Credit Shelter Trust a Grantor Trust?

To be fair, the code does say that a spouse of a grantor is treated as a grantor as well. But I seriously doubt that applies after the actual grantor dies, because they are no longer spouses. In addition, §2056 basically provides for marital trusts. In general the surviving spouse receives all the income from the "B" trust, so it's taxed to her in any case, irrespective of grantor trust rules.

Agreed.

Stu