The Utah based company that did the polling has ties to BYU. It's hard for me to imagine a company with Mormon leanings willingly allowing itself to be paid by Mitt's rivals to attempt to tar a Mormon candidate -- on the basis of his Mormonism.
I seriously doubt a tax exempt religion that has taken great pains to not get involved with candidate politics* would have its students or employees call people and tell them anti-mormon talking points. It would be a little like Rome calling people and asking them if they knew Catholics believe the Pope is God.
(The church never has endorsed a party or a candidate - they have however been involved with Gay Marriage, Lottery, ERA and Alcohol laws - aka issue politics)