I finally sprang for an update for iWork, Apple's suite of office software. I got the install package in today. It installed flawlessly on my laptop. When I went to install it on the Mac Pro, though, the CD wouldn't mount on the desktop like it usually does.
It took me about 10 minutes on the phone with AppleCare before I discovered the problem: I had Windows running in Parallels Desktop. When Parallels Desktop has something booted in it, and is configured to pass the CD drive through to the guest, it never mounts on the OS X desktop. I only found it by shutting down Parallels Desktop on the way to trying to reboot the machine to see if it'd boot off the install DVD.
Damn, I'm embarrassed.
Oh, for the uninitiated: PEBKAC is an acronym for Problem Exists Between Keyboard and Chair.