The vbo_clean branch was merged into Mesa over the weekend, which adds hardware-based VBO support for those with newer ATI Radeon graphics cards. According to one developer, this has already caused a major frame-rate improvement with the open-source ATI stack compared to just a week ago. This new ATI support does require an updated DRM and graphics driver stack to benefit from its TTM memory management. A description of the technical side of OpenGL Vertex Buffer Objects is available here. The OpenGL occlusion query support will also be of benefit to a number of ATI Radeon customers and its technical overview can be found here.
Look for more changes in Mesa 7.6 before its released sometime later this year.