Even with her decision not to seek the Presidency in 2012, Palin remains a powerful motivational and ideological force for a huge part of the conservative base, as indicated by the response to her at the Searchlight Tea Party in Harry Reid's home town, below...
This means she still has a role in the ongoing election, where Hermain Cain's popular support is posing an unexpected challenge to the GOP's intended annointment of Mitt "It's My Turn Dammit" Romney as the party's nominee.
The thing is, she has only one bullet for her endorsement gun, one shot at leading her base to support another particular candidate.
So when does she pull the trigger? Note, I am presuming she is not considering pulling it for Romney. Romney is not popular with her base, and contrary to Ace of Spades, her base is largely supporters, not followers. There's a difference. Also, I don't see her endorsing Perry unless he can get his campaigning act together and do a better job of convincing the country he's the hotshot governor we thought he was.
She could pull it before the primaries. Select who she believes to be the strongest, truest conservative candidate, that would put a lot of money and workers behind that candidate.
That would also be a direct challenge to the corrupt, tired RNC to pull out all the stops in favor of Romney. GOP leadership is already screwing with the primary schedule to try and prop up Mitt, and the GOP's poobahs have demonstrated a truly remarkable ability to backstab, viz. Rove and Castle, Dede Scozzafava and the NYS GOP leadership, or the RNC's championing of absurd outsider candidates like Fiorina and Whitman. I suspect, given the provocation, though, they could outdo themselves this time. The earlier she endorses, the more time it gives them to foul the nest.
She could pull it during the primaries. That would give her more time to see which candidates were prepared to make a fight for it, and minimize the time the RNC would have to screw things up. But it would mean writing off early primary states whose delegates might be needed down the line.
That would be a viable option if, as was assumed by the RNC and Romney's backers, the earliest states, FL, NV, NH were a lock for Romney. But Cain is leading in the polls now in FL, and rising in New England; those early delegates could serve to build momentum for Cain, for instance, in the later states. So it becomes a juggling act of what's to be gained and lost.
Or, and this is a counsel of desperation, she could withhold her endorsement until after the nomination. I say counsel of desperation because this presumes Romney is the candidate. If one of the base's candidates were to be nominated, her endorsement would not strictly be necessary, the base would already be energized. In that case Palin would be smartest to turn her endorsement and assets to House and Senate races in key swing states. But if Romney is nominated, she would be facing the task of motivating her base to support a candidate they do not like now and would like less for losing to him. It would be difficult for her to do this; as I said above, she has supporters, not followers who will slavishly march where she points. Sarah Palin is a leader because the parade likes the way it is going with or without her. In this case she might also be smartest to turn her efforts to the House and Senate as a cushion against Romney's failure... or, perhaps worse, success.
I'd like to see her endorse before the primaries, and I'd like, frankly to see her endorse Cain as the last real conservative standing who is not a part of the old 20th-century GOP machine and dynasties. But as per usual I assume Sarah Palin will make up her own mind for her own reasons... and likely surprise us all.