Mesa 9.0 is a new development release. People who are concerned with stability and reliability should stick with a previous release or wait for Mesa 9.0.1.
Mesa 9.0 implements the OpenGL 3.1 API, but the version reported by glGetString(GL_VERSION) or glGetIntegerv(GL_MAJOR_VERSION) / glGetIntegerv(GL_MINOR_VERSION) depends on the particular driver being used. Some drivers don't support all the features required in OpenGL 3.1. OpenGL 3.1 is only available if requested at context creation because GL_ARB_compatibility is not supported.
tbd
Note: some of the new features are only available with certain drivers.
TBD -- This list is likely incomplete.
The legacy/static Makefile system (ex: 'make linux-dri') has been removed. The two supported build methods are now autoconf/automake and SCons.