I applied and was accepted to WashU MSTP last year (I almost went there).
Anyways, during the revisit, I specifically remember Brian Sullivan (mstp admin) say that an undergrad degree is not required for the MSTP program.
He didnt say how many people actually get accepted to WashU without an undergrad degree, but from his tone I implied that they didnt really seem to care about it that much. He was very nonchalant about it.
neuronix you might be right that they accept almost nobody without a degree, but from what I understand, it really doesnt factor into their decision.