I like McNair a lot, but I think what he's done is simply show how completely, utterly terrible Kyle Boller was - McNair isn't turning the ball over, period, he's not really done much other than that. Not being awful doesn't deserve an MVP award.
Brees, on the other hand, is having a monster season for a team that was picked to finish dead last - Sean Payton should also be the coach of the year, their playcalling and use of Bush/McCallister has been tremendous.
LT is great, but his yardage numbers are actually down from last year - TD's are amazingly high, but then again they are the highest scoring team in the league. He gets my 2nd place MVP vote.