By RICK PORTER Copyright 2003 Zap2it LOS ANGELES -- The jokes Katie Couric usually tells are a little removed from those in a talk-show monologue. The Today show co-anchor offers up this gem as one of her favorites: What did the snail say when he got on the turtle's back? Wheee! Granted, Couric's audience -- her children -- is a little different than that of The Tonight Show's. It's therefore a good thing that she'll have writers to supply her with jokes when she and Tonight Show host Jay Leno switch jobs today. The stunt is part of a "Trading...