As of today, and forever forward, Movable Type is open source. This means you can freely modify, redistribute, and use Movable Type for any purpose you choose.
It looks like the open source version (MTOS) has all the features and functionality of standard Movable Type 4 edition (MT4), but also includes other improvements and bug fixes. It sounds like the code was forked, “open-sourced”, and now released. The current MT4 product, along with the enterprise addon software and support packages will continue to exist (and will be paying the bills at SixApart).
My upgrade to MT4.0 is still half-baked, but I really need to finish it off. I’ve already got a Pro license for MT4, but will want to check out the MTOS version as well before completing this upgrade process.