Well, Palmer's still the best of the quarterbacks. He is simply hurt.
As for Leinart, I started to get into this in an earlier post on this thread but changed my mind. I'll do it now:
Leinart was a tremendous college quarterback. Given how well he played, I don't think anyone can really fault Carroll's decision to start him over Cassel. However, one of the big questions about Leinart coming out of college was arm strength, with questions about him being able to throw the sideline outs and long lobs, and those throws are a lot more important in the NFL than they are in college. Despite what a lot of people on this site were insisting to be the case during training camp, Cassel's arm is more than capable of making all the throws needed in the NFL.
Also, I'm a believer in a theory about NFL quarterbacks that I've cobbled together over the years. My theory is that, for most quarterbacks, the first 1.5 - 2 seasons on the NFL field will make or break a quarterback. By that, I mean that the habits formed in the formative games are very difficult to unlearn. I look at David Carr, for example, and I see a player who was mentally broken by the beating suffered in Houston. Steve McNair, on the other hand, was able to learn the game from the sidelines, so he was ready to deal with the game once he hit the field.
By sitting for a while, and learning the Patriots style of offense without getting beaten to a pulp, Cassel has had the NFL game instilled into him to the point where it was instinctive and just needed honing by actual game experience. Leinart broke into the league under a lesser coaching staff and that's likely to make for a huge difference throughout his carreer.