First off, a release plan is being formed for Mesa 7.8. Mesa 7.7 was released just before Christmas but there is already a great deal of work that's ready for Mesa 7.8. Mesa developers are now looking for Mesa 7.8 to be released by the end of March, which will also come just before the release of X Server 1.8 in the X.Org world.
Mesa 7.8 will bring many Gallium3D improvements (tons), improvements for video playback, a new Intel i965 Gallium3D driver, the mentioned EGL state tracker, a plethora of improvements to the ATI Radeon R300 Gallium3D driver, and tons of other work.
As was mentioned in the mailing list thread that was started to plan out Mesa's next release, there are also more OpenGL ES / EGL changes coming, cleaned-up texture image code, and enhancements to Gallium3D that are needed to support OpenGL 3.0 (and the subsequent 3.1 and 3.2 specifications).
Mesa 7.7.1 will also be released around the same time as Mesa 7.8.0. Mesa 7.7.1 will carry additional bug-fixes and stabilization work atop the Mesa 7.7 branch. Additionally, Brian Paul mentioned that once the OpenGL 3.x support is in place for Gallium3D, this release will be known as Mesa 8.0. However, no release plan has yet been determined for Mesa 8.0 as it's all contingent on when OpenGL 3 support is in place. Judging though by the pace at which Mesa and Gallium3D have been advancing as of late, Mesa 8.0 should hopefully be here in 2010.