As it happened, I used the Mac OS X DVD to update a machine to 10.5.6, and tried to use that computer as part of my compile farm (using Xcode’s Distributed Build feature). It showed up as incompatible with the others. Apparently Mac Box Set includes 9G66, while the combo update installs 9G55. I didn’t realize Apple was distributing multiple builds of 10.5.6. Normally no one would really care, but distributed builds need perfect configuration matches.
So I probably won’t be able to use this machine in my compile farm until the next Mac OS X update.