First, this is about beta sites that might not even exist later, but imagine they will for the sake of this question.
Taking the latest 10 questions on the Unix Stack Exchange, I see 3 questions related to Mac OS X:
Software developer switching from Linux to OS X, what are the gotchas?
How can I (and should I) use my Linux file server as a Time Machine backup server for my Macs?
How can I tell what version of OS X I’m on from the command line?
From my point of view, all 3 could also fit on the Apple Stack Exchange, the 1st one could also be on SO, the 2nd and 3rd could also be on SU.
How we will deal with this type of situation? When should we close a question as "Belongs on [...]"?
I already see identical questions asked on 2 or 3 different sites (by the same or different users). Is there anything planned to link these questions or something like that? I mean something else than the already existing "Linked" feature. Something to include answers from site X in site Y or something like this.