Well, it is a little tricky.
If you really want distinct sites, you can't do things like
1) mount notiifcations in the subsite
2) have the same email in more than one subsite
and other things I can't think of.
One other issue is force_host_p which should be modified for a list of canonical hosts for each mapping (or host not map should support it directly.
Otherwise it is simple to setup just go to admin/ for the Main subsite and click Host Node Map and fill out the form. You can actually map to any site node, but you really need a subsite so people can login etc.
I think this is an area where we should work through the issues and fix them.
