Jordan Justen of Intel published the initial Interface Block support, as mandated by GLSL 1.50. The shading language 1.50 version is for matching OpenGL 3.2.
As explained at OpenGL.org, an Interface Block is a group of GLSL input, output, uniform, or storage buffer variables.
There's 12 patches making up the GLSL Interface Block support that introduce around 400 new lines of code to Mesa. The patches are currently floating in this Mesa thread.
Hopefully Intel and other Mesa developers will have OpenGL 3.2 + GLSL 1.50 support firmly in place within the next five months so that the next release will be Mesa 10.0 with official GL 3.2 compliance.