Macs aren't backwards-compatible, i.e. they can't run versions of OS X that pre-date the version with which they shipped, but they are forward-compatible as far as Apple allows them to go.
I'll get a tad anal on that comment and remind the masses that macs can very often go back a point or two from the version of os they ship with. Thus a mac that ships with 10.8.3 may be able to boot off 10.8.2 or even 10.8.1. A mac that shipped with 10.7.0 may very well be able to boot off 10.6.8. On a few occasions we've back-installed a machine that just barely shipped with a newer release of mac os to the latter, because people were using apps that wouldn't run on the new os or required a paid upgrade.
This also leads to a similar issue where a mac that shipped with 10.6.5 may or may not be able to boot off a retail 10.6.3 installer disc. But if you can install the os by using the mac in firewire target mode booted off an older mac, and then run the combo updater to bring it up to 10.6.8, it will then boot on its own and resume the updates etc.
