Anonymous sources?

Michael Joseph Gross gave a lengthy (for TeeVee) interview with CNN. They discussed his article, "Sarah Palin's Sound and Fury," which was published in Vanity Fair:



On the use of anonymous sources, this part of Gross' article was briefly discussed:

One person who has been a frequent houseguest of the Palins’ says that the couple began many mornings with screaming fights, a fusillade of curses: “ ‘Fuck you,’ ‘Fuck this,’ ‘You lazy piece of shit.’ ‘You’re fuckin’ lucky to have me,’ Sarah would always say.” (This person never saw Todd and Sarah sleep in the same bed, and recalls that Todd would often joke, “I don’t know how she ever gets pregnant.”)

"This person never saw Todd and Sarah sleep in the same bed, and recalls that Todd would often joke, "I don't know how she ever gets pregnant." Who is the source?

In "Me and Mrs. Palin," Levi Johnston appears to state:

After the nomination, Sarah and Todd wouldn’t go anywhere together unless the cameras were out. They’re good on television, but once the cameras would leave they didn’t talk to each other. In all the time Bristol and I were together, I’ve never seen them sleep in the same bedroom. (I don’t know how she got pregnant.)

Apparently, Levi hung-out with Todd, or hung-out within earshot, or talked with someone who did.