Nexus 10 "shareable" part of Android 4.2?
Any ideas whether this feature will be part of the core of Android
  4.2, or if it's just for Nexus 10? 


Multi-user support in Android 4.2 will be for ALL TABLETS. So yes, all tabs with Android 4.2 Jelly Bean will have multi-user support.

Google is officially supporting this only for tablets as of now. However, I'm very sure that custom ROM makers will port it for phones. In fact, there are already a bunch of custom ROMs which do this.


  I wish I got this on my Nexus 7.


Nexus 7 is a tablet, so when you upgrade to 4.2, you should get multi-user support.