Can a former candidate for president or vice president face criminal charges? You bet. Look at John Edwards right now. He is accused of accepting illegal campaign donations to pay off his mistress.
Could Sarah Palin face criminal charges? Of course she could. But only if a law enforcement agency decided to bring them. And if the Mat-Su medical center or its employees were found to have conspired with Palin in a fraud, could they be criminally liable? I think so.
So what charges might Palin face? Well, fraud comes to mind. According to Wikipedia: "In criminal law, a fraud is an intentional deception made for personal gain ..."
Faking the birth of Trig meets the requirement of "intentional deception." What about the "personal gain" part? That is tricky. Certainly the U.S. Justice Department (for example) could bring strong case if it could reasonably argue that the baby hoax was done specifically for the purpose of helping make sure Palin would become the vice presidential candidate of the Republican Party.
I want to consider the question of fraud in two parts. First, with regard to the moral question; then the legal question.