This is something not to be missed: The Hardball Times have an article breaking down Prior's motion from 2001 and now with video. They go frame by frame and show you the differences between then and now.

If you only have time to skim, be sure to scroll down to the last frame where they show the follow through. It's no big surprise that he isn't throwing nearly as hard this year.

From the article:

I realize that most of us have been taught to be deliberate and "under control" with our mechanics. You were taught the "tall and fall" and learned it well. You were blessed with the arm strength and learned the proper arm action to put 95+mph force on the ball. However, you need your body to help your arm. You need to learn how to use your body better. I have one suggestion, and it is a major adjustment.
GET IT GOING!!!

My personal take on it would be that it now looks like he's pitching with a stick up his ass. He used to look like a pitcher, but now he looks...oh, I don't know, British or something.

Perhaps if he could just cut loose and get a little aggressive he might get that velocity back. I've always assumed that he was hiding an injury, but I think all the mechanics issue have simply made him gun shy.

Either way, I miss the old Prior.  Come back, old Prior!  Come back!