I think I've figured this out--and it took some doing. What I've discoverd, both with the WordPress MU Domain Mapping plugin and the WP Multi Network plugin, is that half of the work required to set EITHER up successfully will depend on how familiar you are with how your registrar and host talk to each other (assuming your domains are not registered with your host). Also, my assumption from the beginning was that you had to have domain mapping in place prior to using the WP Multi Network--but that's not the case. From the looks of it, you can use WP Multi Network without WordPress MU Domain Mapping.
A couple of pre-requisites prior to configuring WP Multi Network:
- All the domains you plan to use in your network need to be pointing to your host's name servers. This is pretty obvious.
- Once you've done that and given DNS enough time to propogate, you'll then have to go to your host's cpanel, add the new domains, and either park them or do some sort of the equivalent. If you use Site5, you'll have to park them (see this thread); if you use Dreamhost, you'll have to "mirror" them (see this thread). These threads apply to domain mapping; but as you've discovered, Kyle, the WP Multi Network is just another mapping plugin, so the prerequisites for using it are similar to setting up WordPress MU Domain Mapping.
In a nutshell, if it ain't working for you, then it's probably an issue with your registrar/host's domain settings. But once your domains have propogated and are properly parked or whatever, adding them to your network is pretty easy through the plugin GUI.
Where I'm stuck now is figuring out why blog registrations key off of the mother blog and not their own admin options settings. For example, if you allow blog registrations for one domain in your network but not in the original parent network, it won't work; seems you have to enable them in the parent network in order to be able to enable registrations in the child networks.
And to further underscore your point ...
... this is more of a mapping plugin than anything else - it doesn't physically create anything.
That's exactly right. When blogs get registered, they also seem to only get registered against the parent network, even if a user is attempting to register through a child network. And then it will be up to the admin to assign that site to whichever domain in the network it needs to be associated with. It would be nice if the plugin could allow users to login to whichever network they wanted and have their sites automatically assigned to that network simply based on the domain through which they logged in through--or maybe this is already possible through some option I missed?
But, still playing and learning ...