There have been some well thought-out possible trade scenarios in the previous comments; I do agree that Cassel-to-KC is a distinct possibility. I also want to comment that the scenario of the Chiefs drafting Sanchez and also signing Garcia shouldn't hurt them capwise because I don't think Garcia will cost that much. He's already stated that "I feel I can still contribute. I'm not sure what my role will be with another team. I'm not looking at it like I need to start and be the man. Hopefully, I'll have some opportunities out there. I'm pretty sure there's going to be something out there for me." That sounds to me like a guy willing to be a backup - and accept backup pay.
One thing I believe nobody has mentioned however, is that the Chiefs just announced they will change from a 4-3 to a 3-4 defense next year. That's going to mean that they are going to need to bring in multiple players that can fit the new defensive scheme - which means they probably don't want to be collecting draft picks, not trading them away.
If that's the case I can see them sticking with Thigpen in 2009 and then making a decision after the season whether they want to stick with him, or draft a QB in 2010.