Ian Romanick took to the mailing list last night to reaffirm plans to release on the 27th of November. Normally branching would take place about one month in advance, or on 31 October, but Ian has proposed branching on 7 November without delaying the release. The reason for the delayed branching is so that a few more features can land and that Mesa Git right now looks to be in relatively good shape with few open regressions.
Among the outstanding OpenGL items that haven't yet been merged to master but with patches available include ARB_shader_atomic_counters, ARB_sample_shading, ARB_vertex_attrib_binding, and others.
The plan for 7 November branching with a 27 November release can be found via this mailing list post. As covered now in many Phoronix articles, Mesa 10.0 is shaping up to be a very exciting release, but not without some flaws.