Check out this interesting discussion of the Wonderlic exam given pro football players. There has been much talk of the Wonderlic lately on the heals of Vince Young's difficulties, in light of Youngs's famously low score on the exam. However, there are good historical reasons to doubt whether the Wonderlic is a valuable indicator of QB success:
"They use the Wonderlic as a substitute, but there's plenty of to suggest that the Wonderlic doesn't actually predict success in the pros. For instance, Dan Marino scored 14. Brett Favre's Wonderlic score was 22, while Randall Cunningham and Terry Bradshaw both scored 15. All of these quarterbacks have been, or will be, inducted into the Hall of Fame. (In recent years, Favre has surpassed many of the passing records once held by Marino, such as most passing yards and touchdowns in a career.) Furthermore, several quarterbacks with unusually high Wonderlic scores - players like Alex Smith and Matt Leinart, who both scored above 35 on the test and were top ten picks in the 2005 NFL draft - have struggled in the NFL, largely because they make poor decisions on the field."
There are also sound psychological reasons to doubt the Wonderlic, as Jonah outlines. The position requires far too many instinctive, unconscious reactions which bear little resemblance to solving Wonderlic problems. As most who have played the game will tell you when you ask them what they were thinking during play X of the game, the answer you'll get is "I wasn't thinking anything". It's a good read.