Not quite. The Code says that any powers held by the spouse of a grantor are deemed to be held by the grantor, but the attribution doesn't flow the other direction. Once the grantor is gone, then section 678 is the only grantor trust section that can continue to apply because there is no longer any grantor--the surviving spouse can only be treated as the grantor if he or she has the power to distribute discretionary amounts of income or corpus in himself or herself.
This is true for the marital trust (if one was set up under the estate plan), but not necessarily true for the credit shelter trust (which was the subject of the OP's question). The credit shelter trust could bypass the surviving spouse entirely. It's not the usual way to set it up, but if the surviving spouse is independently wealthy and the deceased spouse's kids need the funds, there is no tax requirement to name the surviving spouse as a beneficiary of the credit shelter trust.
--Chris