It was in early March that the question arose of merging the Mesa floating point branch to master. Developers pushed on for this OpenGL 3.x requirement to be merged and then had also proposed that S3TC texture compression be merged too (this though was rejected since it can cleanly live as an external library to Mesa). By mid-April, floating point support was merged and by default is hidden behind the --enable-texture-float build-time flag.
Mesa 7.11 hasn't yet been released, but it will be the first official version carrying this floating textures support. It's disabled upstream and is expected to be disabled by default in most of the major Linux distributions as well due to the patent/legal concerns. It's likely that some third-party package repositories for Ubuntu, Fedora, etc will end up shipping Mesa packages with the option enabled. Users can also build their own Mesa if they so desire.
Following this Arch Linux bug report and a forum discussion with references to Phoronix, Arch Linux decided to carry the enable-texture-float flag.
The reasoning for enabling this support is that some Linux game engines won't work without the support, namely the Unigine Engine. While it's true the Unigine Engine can run on Mesa with this support enabled if you're on Git master, at this time regardless of the hardware driver, the performance is at a slide-show pace and it will likely not be really usable by open-source drivers any time in the near future. If you use the open-source drivers and think otherwise, you can participate in the Phoronix-Unigine OilRush giveaway.
This Git commit to the Arch Mesa package updates the source-code to as of Git yesterday (31 May) and adds in the --enable-texture-float argument.