How Do You MU?

A recently-started topic at the WPMU forums concerns scalability. The original poster asked about scaling WPMU to a million blogs. The first reply was about WordPress.com, and the next was about Lyceum. I’ll be following this topic with interest.

I just saw at Boingboing a link to Lyceum, “a powerful and easy to use multi-blog WordPress that could be used for installations with 2 or 200,000 blogs.” Given that WordPress is free/open source, anyone is free to use and change the code, and that’s what the Lyceum team did.

My immediate question is: how does Lyceum relate to WPMU? In particular, why do Lyceum? Why use it, rather than WPMU? How does the multi-user-ness of Lyceum differ from that of WPMU?