The Movable Type Open Source Project was announced in conjunction with the launch of the Movable Type 4 Beta on June 5th, 2007. The MTOS Project is a community and Six Apart driven project that will produce an open souce version of the Movable Type Publishing Platform that will form the core of all other Movable Type products.
movabletype.org: Welcome to MTOS: the Movable Type Open Source Project
Six Apart is trying to build a open source around MT4. May work, but it is a crowded arena. Note to self: requires lots of Perl and MySQLBlogged with Flock