A couple of days ago I was watching with my three-year-old daughter the Veggie Tales movie, "Jonah" (a cute and funny movie even for oddball adults like me, actually). While watching it, I was struck with the fact (which I guess I had seen before in the Book of Jonah itself, but hadn't noticed before) that Jonah NEVER really had a heart that was right. All through the book, he operates from ungodly motives of self-preservation, cultural prejudice, and general selfish malevolence. The end of the book finds God rebuking the prophet for caring more for his own comfort (afforded by the gourd God caused to grow as a shade for Jonah) than for the welfare of the people of Nineveh "and much cattle."
What really struck me was the fact, so often and so sadly observed, that a person may in fact be MIGHTILY used by God without even himself having a pure heart. Could this sort of thing be what Jesus was referring to in Matt. 7:21-23?
So the question of whether or not John the Baptist was regenerate in his mother's womb seems moot with regard to his divinely ordained role as a prophet of God. The same God could use a donkey to speak for Him if need be.