Why would the announcement be on the Bob and Mark show? There must be some significance to Bob and Mark for Sarah to use their show for the big announcement.
A promise she made that they would be the first to know. Sarah has a long memory and a remarkably strong habit of keeping her word.
She promised them (more than once) that she would announce on their show.
http://us4palin.com/gov-palin-outlines-palinomics-on-bob-mark-show/