Movable Type is now open source software, released under a GPL license. Their status page has details on which pieces of their open source support infrastructure are operational, including the SVN repository, nightly builds, and bug tracking system.

MTOS is identical to Movable Type 4.0. Six Apart will continue to sell licenses with support contracts and paid benefits (some plugins and templates), but the main software and default template set—everything you need to set up a blog—are all free and released under the open source license. 6A has a slightly tricky branding problem on their hands describing the open source version of MT and their paid "versions," and they've decided to call this "MTOS" (as opposed to "MT"), even though the main software (minus paid-only extras) is and will probably always be the same across versions.