I have two separate WP sites set up using the multi-domain & domain mapping plugins.
I may have either plugin configured incorrectly because the main site operates without the "www" portion of the URL. (Adding "www" to it causes it to redirect to the non-www version, which is fine I guess.)
However, the second website only works by using the www portion. When I use the URL without it I get redirected to the main website's Network Registration page (when not logged in), or the main website's home page (when logged in as Administrator.)