Very interesting to look back on this at this point. All of these guys had great regular seasons. Let's examine the candidates after their playoff performances so far.
Barber - 69 total yards and 0 TDs as his team was blown out at home.
Palmer - Knocked out of his first playoff game after one play.
Brady - Great game in first round win over Jacksonville. Poor game in loss to Denver in the second round.
Alexander - Knocked out of first playoff game early. Still alive, but team won reasonably easily without him.
Manning - TBD, but still alive.
Now, I personally have always felt that MVP awards should be given once postseason is complete. In all of the major sports, it is very rare that a player from a non-playoff team is strongly considered for the award, which simply underscores that an MVP is someone who usually strongly contributes to the chance for postseason success... why not actually include that postseason success or lack thereof? Why do we really need a regular season MVP and a playoff MVP? Postseason success is supposedly what it's all about, so how meaningful is an MVP award based only on regular season? (And for that matter, not even on all games of the regular season, based on when it is voted on.)
It is clear to me that, of the 5 candidates who were well understood to be the true MVP candidates for the regular season, Manning is the one who will most likely have had the most valuable season to his team when the postseason is complete.
That said, if my preferred method was used for this voting, there would still be other candidates alive: Roethlisberger, Steve Smith, Delhomme, Urlacher, Hasselbeck, Plummer, and maybe Alexander if he comes back strong in the NFCC and the Seahawks make the Super Bowl.