It would make her look indecisive.
I really doubt it. Sarah could easily explain it. She decided not to run the first time because she thought that the conservative banner would be held by Perry or Cain but their campaigns have faltered. Rather than see conservative positions unrepresented in the primaries, she had to get back in for the good of the country.
I don't think that she'll have to get back in. Unless something much more substantive than the allegations heard so far, Cain can probably persevere.