SlideRocket comes out of public beta today with some new features (TechCrunch post).
Given increasingly heavy PowerPoint files (images, video), I think the only future solution for collaborative presentation design is an in-the-cloud model. Getting people to learn an new interface as an alternative to PowerPoint though is difficult. And, Google Docs, Microsoft's Azure, are some pretty heavy competitors to beat...
SlideMagic: a platform for magical presentations. Free student plan available.