Looks like submitted code must have copyright transferred to Six Apart:
http://www.movabletype.org/opensource/mtos_contribution_a...
In any case I'm glad they decided to release MT. I used it back when it was freeware, then
migrated over to WordPress when MT was locked down. Since WordPress never got around to
supporting multiple blogs with a single install, I ended up having to run/update 3-4 instances
of WP, and with their recent habit of having a CRITICAL SECURITY UPDATE every month or two,
this has gotten to be a real hassle.