In general, I agree with Bryan Berry's take on "educator-driven development," in fact, it is what SchoolTool's process has evolved to and is working well for us. Mark Ahlness has a wonderful post today illustrating another example of this process, using XO's in his classroom in collaboration with developers from the University of Washington.
It is important to remember that open source development of learning software doesn't and shouldn't just come from grassroots hackers. It has to be a multifaceted approach, lowering development barriers so that educators can contribute code themselves, engaging local government in funding development, working closely with existing projects which promote open source in education, evangelizing the platform to technology educators and researchers accustomed to proprietary platforms, making it easy for existing software that isn't education specific to be used in schools, and more. OLPC has considerable room for improvement in all those areas.