I Was Partly Right
My claim that leftists would accuse Sarah Palin of violating conservative principles, when all she was doing was violating leftist stereotypes of conservatives was right but I was wrong about what their accusation would be.
The mistake of the leftist activists was in thinking everybody else would define hypocrisy as doing X and preaching Y whereas social conservatives are more likely to define hypocrisy as doing X and denying it. We must recall that a large fraction of evangelical preachers will include something like “I, too, was a sinner.” in their sermons. (For that matter, the activists ignored the fact that Ms. Palin's oldest child was born less than nine months after the wedding.) In other words, she would have little motive to cover up a temporarily-illegitimate grandchild (the social conservatives would forgive that) and a strong motive not to do so.
A conspiracy theory I haven't seen yet
What if the rumor had been planted by a right-wing agent? It would make leftists look like idiots when the truth was revealed, provide an opportunity to announce the real pregnancy, and remind social conservatives that leftists don't understand them.