Like my compatriot, Jason Snell, I'm more than a little keen on Ambrosia Software's Snapz X Pro 2.0. I use it extensively to create stills and videos for my monthly Macworld video tutorial, Breen's Bungalow, and, of course, for screenshots that appear in the Mac 911 column. The latest update is a remarkable piece of work -- allowing you to capture large swaths of video at high frame rates.
While beta testing Snapz Pro I found that it did have some difficulty capturing stills of iPhoto 4 Slideshows -- you'd hear the familiar "click" indicating that Snapz was ready to do its business, but the Snapz interface never appeared.
I found you could work around the problem by invoking Snapz and then pressing Command-Option-Escape. This forces the Snapz interface to the foreground, allowing you to capture the screen. The regrettable side effect is that once you've taken your shot, iPhoto quits.