Does this rise to the level of blackmail? Or is it only a crime when it happens to David Letterman?
Not Carrie Prejean. Not others.
In the technical sense, no. She was smart. She got a lawyer, who scheduled a "press conference" but was nondescript about exactly what was going to be discussed. It could have been a press conference on a new business venture, or endorsement deal.
So long as she, nor her representatives, didn't initiate contact with Tiger, she didn't break any laws. Extortion is an active crime, not a passive crime - like willful disregard. You have to actively engage (called coerce) someone in order to satisfy the definition extortion. Granted, this comes close, but it doesn't get there.
Prejean, OTOH, I believe probably was extorted if the reporting is accurate. She was told do something, or damaging information would be made available. That's coercion.
Given the nature of the pre-nup, I would describe this report as a re-negotiation. Happens everyday in the sports world.