Comparing the 2007 with the current poll numbers, I think there is still plenty of time and room to improve for serious candidates like Pawlenty or Huntsman who are now polling in the single digits - Huckabee has shown that it's possible. A candidate like Cain, however, has probably reached his peak. He seems to have been losing ground in the polls recently.
Hunstman has already declared that he'll skip Iowa, so the whole conversation is moot.
Pawlenty has nowhere near the charisma and charm of Huckabee, nor the natural constituency to strongly support him.