Well, Peyton hasn't deserved the MVP the last two years, and he got it anyway.
This season, Peyton is a middle of the pack QB on a middle of the pack team. He doesn't even deserve a Pro Bowl spot. That could all change, and I have no doubt he is a top 2 QB overall, but just looking at his play this season, I don't see why Cassel wouldn't be ahead of him for the Pro Bowl.
Rivers has had a great year, but he's also had an abnormally easy schedule in terms of pass defenses (and a similarly easy schedule to what Peyton has) - whereas Brady, for the 2nd straight year, has faced a truly brutal series of pass defenses. Where Rivers has really made his mark is by producing no matter who else was on the squad with him. That says something. I used to hate the guy and think he was just another whiney-Manning-face type, but he's really gutsy, and ever since he played us without an ACL in the 07 AFCCG, I've respected him.