If you installed from testing then you can just downgrade the appropriate packages. The arch wiki has an entire page dedicated to the process, warnings, and how-tos of downgrading packages: https://wiki.archlinux.org/index.php...ading_Packages
Keep in mind...
You might be better off asking on the Arch Forums specifically, I run Fedora now otherwise i'd be willing to walk ya through it, but the actual Forums might be better off at giving you a dependency list for EVERYTHING that needs to be downgraded beyond just Mesa.Consider the dependencies of each program. The required libraries often change with each version, and the functionality of associated files may be completely different from previous ones. The solution will require changing these to earlier versions as well.
Now, all that assumes you installed actual arch packages... if you built and installed by hand then things get complicated, and again...ask the Arch forums.
Chances are, both OpenGL 3.2 and 3.3 will be finalized in the same Mesa version, which would make it 10.
I highly doubt the Mesa folks are going to release Mesa 10 for OpenGL 3.2 AND Mesa 11 for OpenGL 3.3 next november and try supporting two concurrent streams, that just wouldn't make sense. The gl3.txt page makes it seem like the work for OpenGL 3.3 is pretty much done, so I don't see any reason why 3.2 and 3.3 will land at different times.
It will be nice once they finalize the entire OpenGL 3.x stack.
BTW, where are Fedora packages of this? Mesa 9.2.0 rc1 solves an important issue with EGL + r600g (vsync is broken), so if I can get those updated packages, I'll finally be able to use KDE 4.11 with EGL and v-sync. So Mesa 9.2.0 is really important for us.