For this release, which originally was going to be known as Mesa 8.1 until they effectively reached OpenGL 3.1 compliance (upon supporting a new GL version they tend to bump the Mesa major version number), it should be out in one week's time.
Originally Mesa 8.1/9.0 was going to be out in September, but branching was delayed so more features could land. Mesa 9.0 was branched with some really great features, including GL 3.1 for the Intel driver, and now it's about ready for release.
Romanick on Friday evening sent out a new mailing list message with some patches he's asking about cherry-picking onto the 9.0 branch. He hopes to do those by early next week and then release Mesa 9.0 officially next Friday, 5 October.
So if all goes according to plan, on 5 October will be the next official Mesa release. Performance benchmarks of Mesa 9.0 plus overview of changes/features since Mesa 8.0 from the start of the year will be published on Phoronix in the coming days.