The ARB_stencil_texturing extension is what's now supported by the latest Mesa 10.2-devel Git code for Mesa after previously only the API was in place without any Mesa GPU driver support. As explained via the OpenGL.org registry, the extension allows allows texturing of the stencil component of a packed depth stencil texture.
Unfortunately for Intel users right now, this extension is only hooked up for the upcoming Broadwell processors. As explained in the commit message, "On earlier hardware, we had to implement math in the shader to translate Y-tiled or untiled coordinates to W-tiled coordinates (which is what BLORP does today in order to texture from stencil buffers). On Broadwell, we can simply state that it's W-tiled in SURFACE_STATE, and adjust the pitch. This is much easier."
The good news for pre-Broadwell owners is that Topi Pohjolainen of Intel published on the Mesa mailing list in late February a set of patches implementing ARB_stencil_texturing support for Sandy Bridge and newer hardware, which will hopefully be merged soon.
At least Broadwell is looking good on Linux for when it debuts in a few months time, but officially the OpenGL compliance for the Intel Mesa driver is still at OpenGL 3.3 with a number of GL4 extensions being supported but OpenGL 4.0 support likely won't be officially reached until at least later in the year.