Mesa Can Do EXT_texture_compression_RGTC
The EXT_texture_compression_RGTC extension work is supported in core Mesa, the software Rasterizer, and with Gallium3D (works with the ATI R600g driver). This compression code is ported from Roland's DXTn texture work. While this is not the S3TC texture compression work, this OpenGL extension at least doesn't appear to be covered by any patents or other protected intellectual property.
The specification for EXT_texture_compression_RGTC can be found on the OpenGL registry. This extension is not new but was approved back in 2008 and was written against OpenGL 2.0.
This extension introduces four new block-based texture compression formats suited for unsigned and signed red and red-green textures (hence the name "rgtc" for Red-Green Texture Compression).
Too bad though it will still be a long time before Mesa reaches any sort of parity in terms of OpenGL extension support or even provides proper OpenGL 3.x/4.x support, with the Gallium3D/Mesa drivers still effectively living in the OpenGL 2.1 world.