As a long time MT user(*) and professional I really welcome the initiative. Movable Type is my CMS platform of choice and represents a significant chunk of my services revenue. Having a renewed community impulse around an open source version can really improve both the free and the commercial products.
This said I think Brice Dunwoodie makes some good points about the MTOS fork, notably how far the fork will split from the core Movable Type. Considering the strong relationship between the OMSG founders and the MT community (Six Apart included), I'm not too worried.
(*) You're a long time MT user if you know why Melody. The joke's on Nelson. ;-)