James Farmer who is the founder of Edublogs.org, one of the largest examples of WordPress MU in action has recently launched a new service called Blogs.mu that enables anyone to set up their own blog network. James describes Incsub as WordPress.com in a box. Instead of managing a WordPress MU install yourself, Incsub takes care of all the heavy lifting for you and even includes ways for you to run your own advertising.
Problogger.net has a great interview with James which dives in a little deeper into how the service works.