We test, because OS X is not backwardly compatable enough.
We currently have 1 enterprise product that DOES not work on OSX 10.5, but does on 10.4 fine.
10.5 also broke our centralized authentication process. So if you run unmanaged Macs then yep its all fine, but that is not running an enterprise setup, that's just running a heap of Macs.
Its not the same, and it doesn't scale well
We more than halved our support calls times on Macs when we rolled out a centrally managed and supported managed operating environment,
it would have been more, but apples last minute upgrades, and breaking of backward compatability
now require significant more testing.
We support Linux, Win XP, and OS X, and the linux and PC's are the cheapest to support, and have the best backward compatability.
Just go to
http://www.macintouch.com/leopard/compat.html, to see what broke and what only kinda broke, (and its not a small list), we were affected by a number of these issues.