The problem with OpenBSD is that it does not have as many partisans (read, *really* good geeks working on neat applications). No doubt Apple will port a number of really cool applications to OpenBSD but it will, in the end, suffer the same fate as the PPC Linux versions for Apple hardware: its releases will lag the main PC platform Linux releases...
the new stuff will generally show up on the PC platform versions of Linux FIRST. Those applications that DON'T show up on the PC linux first will be ported or cloned soon enough.
Bottom line: since the PC platform stuff is SO, SO much cheaper than the Apple stuff, and the software is CLOSE ENOUGH, Apple will have a marketshare problem and be stuck as an also ran.
What makes Apple stand out is Apple software you can't get on the PC platform on software, and software that doesn't play well together on the PC platform.
I wish them luck in making the transition to OpenSource: it is NOT at ALL an easy road for a computer maker...
There's a lot of BSD that got stripped out of OSX, but I think it's fair to look at OSX as if it were Apple's proprietary UNIX.
BSD, and NeXT too.
Best thing about OSX right now is Microsoft OfficeX. Man, is it nice.