There are plenty of top dogs that work for food or a toy. Someone here is going to want specifics, so, from MY own eyes.
The 1988 World Champion Police (WPO) dog did his obedience for a tennis ball. I have also see two of our former WUSV team dogs work for a tennis ball in obedience. For those that don't know what the WUSV is, it's basically the World Schutzhund Championship for GSD's, [whom dominate the sport anyway]. Just my opinion, but I would consider making this team the highest of honors. I will not mention names, as I do not want to talk about someone else's training, without their permission.
Sneaking food, or a toy, onto the trial field has been transformed into an art of it's own. I learned many little tricks from German "Leistung richters". It's done at the Nationals, the BSP, DM, and those ever popular club trials.
http://www.rottweiler.net/rottie/wink.gif