Barbour says that he will make some kind of announcement next month, after the Mississippi legislature finishes its session, and he becomes a more-or-less-official lame duck.
They have all been holding back. Huckabee told me that one of the lessons he learned from last time is that the whole thing started too early, and that the candidates were pretty fatigued (not to mention out of money) by the time voters started paying attention. I also think that they are all trying to figure out who is in and who is out before making their own moves. Palin, for instance, would really change the dynamic if she got in.
Newt and Pawlenty seem furthest down the road, in terms of setting up their official campaigns. Romney, on the other hand, already has an operation from last time, so he can afford to wait.