I'm with the crowd that wished Palin had announced her intentions, but I get the impression that she doesn't know yet. She might be exploring a presidential bid, but doesn't want to announce that yet because she might see the Senate as a fallback option. Give her a couple of months to get her footing and some speaking fees.
I'm not so sure this is a political move....as much as it is a personal one. As much as I wish I knew "why" she stepped down..... Palin is due her privacy... and she's not obligated to give the media a darn thing. They only want to pick the bones of any surviving relative left in her bloodline.