I'll give you guys a funny story. My wife works with Mark Prior's sister-in-law, and I had a chance to hang out with Mark Prior's brother. To my surprise, he told me that he almost never plays catch with his brother. Why? Because Mark always has problems with soft toss( throws over your head, or bounces one to your shin) Apparently, he is so used to his pitching motion that anything other than his normal routine is a complete disaster. That being said, if you hold out your mitt,present a target, and are not afraid of a 90mph throw, then it's a strike everytime.
So basically some inside info, Mark will be fine. His motion and mechanics are superb.