For reasons probably only Steve Jobs knows, the 10.5.5 update breaks ProPresenter in a major way. It's causing Kernel Panics (those are really, really bad...). Renewed Vision is working on figuring out what Apple broke this time and will post an update when it's figured out.
As always, it's prudent to hold off on installing any updates on a well-functioning system (and especially mission critical systems) until they're proven to work well and not break anything. This includes (and sometimes means particularly) Apple updates.
Apple is somewhat notorious for changing things and not letting developers know ahead of time. So be aware. And it never hurts to have a fully functioning backup of your previous version just in case. See Back the Data Up, Pt. 2. for some thoughts on how to accomplish that.
[Update 9-17-08] It appears this issue is only a problem with MacBook Pros. And there is a work-around for it.