There's a good post on HN about what it cos Apple to make OS X a certified UNIX. Due to the secrecy of the project they had to use non-corporate accounts for committing bug fixes. The amount of changes was pretty big, but nobody excepts the authors can say it came from Apple.

Of course they will commit to open source projects if it suits their needs (if they use those projects). But people have different opinion on what Apple's needs are.