Thanks for announcing this, Jon, and thanks to the Steering Committee for making a decision.
From where I am sitting, CMake is the clear winner of the "build system war". Regardless of how you personally feel about the technical merits, it is hard to argue that Boost as a project and a community is not better off due to this change.
To some people, this seems out of the blue. To me, this seems inevitable. There have been many discussions on the Boost mailing list over the past months (years, even) about having CMake be a requirement for new libraries and using it as the default build system. We, as a community, pride ourselves on keeping up with modern software development practices. In C++, that increasingly means using CMake. We should not be concerned that we will be unable to make the change -- projects far larger and more complicated than ours build on CMake already. That's one of the advantages of making use of tools that have broad support: most of the work is already done. If you imagine two possible futures: one in which Boost uses CMake as the build system and one in which it uses b2, which future do you imagine is better for Boost in the coming years?