While Ivy Bridge and Haswell have OpenGL 3.3 in the forthcoming Mesa 10.0 release, Sandy Bridge is now the left-out child to Intel's Linux developers. Sandy Bridge isn't hitting OpenGL 3.2 compliance for lack of the driver implementing geometry shaders. This comes while Broadwell already supports OpenGL 3.3.
OpenGL geometry shaders could be implemented for Sandy Bridge hardware, but no one has cared enough to do so. Paul Berry of Intel had wrote on Mesa-dev last month, "I believe geometry shaders are all we would have to implement for Sandy Bridge. Unfortunately, geometry shaders work pretty differently on Sandy Bridge, so getting them to work won't be a slam dunk."
While it would be nice to see Sandy Bridge have better Linux OpenGL support (and performance) since the hardware isn't too old, the Intel Linux developers' resources are limited and personally I enjoy seeing the very interesting advancements they are making to the latest and greatest Intel hardware. Plus nothing should be limiting any other independent developer(s) from limiting the GS support in SNB should Intel not be interested in turning out the code.