As our own Mike Rose points out, the license exceptions seem to allow third party plug-in developers to keep their source code private. Developers (including Apple) can distribute derivative work and be exempt from the mandatory source code release clauses of the GNU GPL so long as the exception is limited to Mac OS X and not for use on other operating systems. Just remember: we are not lawyers, and the exception language is convoluted.
Update: Nilay Patel from Engadget adds: Apple isn't affected by the license terms of CUPS, since they own it. A license only affects licensees, not owners. Apple could fork CUPS and close the source tomorrow and no one could do anything about it, although I'm certain the terms of the sale included a promise that Apple would keep it GPL'd for a certain period of time. The exception appears to be geared to printer manufacturers so they can write drivers and not have to open their code -- which is interesting, since the GPL already provides a mechanism for this sort of distribution, called the "mere aggregation" clause.. Disclaimer: Although Nilay is a lawyer, this information is not legal advice or analysis and should not be construed as such. Thanks Nilay.