The senate is no place for executive leaders. Would have also been nice for Reagan to have come to the presidency with a little less experience (age)—but times choose our leaders rather than the other way around. And, as has been asked here, who better for 2012?
The senate is the best place for her to get back on the national scene and support conservative causes. She can hardly go back to being the governor of Alaska. Being a promoter of conservative causes and speaking at conservative events will only take her so far. Being a senator will thrust her back onto the national political scene and dismiss arguments that she is a quitter. Big Media will be unable to avoid or shun her since she will be front and center in the national spotlight.