It depends of course.
If Palin doesn't run and endorses someone then, except for Romney or Bachman, that's the person I would vote for (Palin supporters are not robots). If Palin endorses a candidate and that candidate subsequently chooses either Romney or Bachman as their running mate then I would not vote for them.
In both the above,"I won't vote for them" cases I would write in Palin.
If Palin doesn't endorse anyone then I would vote in a primary election for, in descending order, Giuliani, Pawlenty, Perry-if none of those were on the ballot I would write in Palin.