A nice concept but the way that the instructions tell you that it has been implemented unnecessarily restricts where and how you can use it. In fact it is more powerful than the instructions suggest.
The concept here is an excellent one and with a single blog installed in multisite mode you have a clear separation between the super admin who decides what plugins and themes will be available and the regular admin who has control of everything else.
The problems start when you follow the instructions to try to set up a second blog on multisite. With a first blog set up at wp.felgall.com.au I attempted to set up a second at t.wp.felgall.com.au. I created the sub-domain in my hosting control panel pointing it to the folder where multisite was installed and used the add site option to add the site. The additional database tables etc were all created but the new sub-domain could not be found.
Things got worse when I installed the domain mapping plugin. With that installed the first blog could not be found either even though I hadn't even started in my attempt to map any domains. Reading the source code of one of the files in this module indicated that it is only expected to be able to work when WordPress is installed in the hosting root folder.
The database and file structures for handling multiple sites from the one copy of the code are well designed to keep each blog separate but the documentation on how this is implemented into the PHP is extremely poorly done as it is completely misleading as to what is required for implementing multiple domains.
There is one table in the database that lists all of the sites that are defined and contains values that point to the corresponding database tables and folders. Provided you have the domains correctly defined and pointing to the WordPress folder the code will look up the current domain in that site table to determine where to look for the data. In fact you can ignore the documentation about the plugin needed for multiple domain implementation and use whatever mix of domains and sub-domains as you like and they all work the same way without any special domain mapping plugin being required. The mapping plugin is only needed if you want others to be able to register their own domains.
I eventually got the plugin working exactly how I wanted by disregarding what the documentation and not installing the plugin for multiple domains and instead simply pointing the domain at the correct folder and updating the site info for the site to use the correct domain name. I did have to add define('COOKIE_DOMAIN', false); to the config file to allow access to the admin area for the sites using different domains.