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.
It will be GL 3.2 and 3.3. According to the "Status of OpenGL 3.x features in Mesa" file on freedesktop.org:
GL 3.3 requires GLSL 3.30, not just 1.50. The changes are likely doable, but I wouldn't be too surprised if they'er not done. Hopefully they are, GLSL 3.30 is when it started to get bearable to use.
And according to the GL3.txt document, the GLSL new 3.30 features are basically done. I haven't looked at the code (or maybe it's in a branch somewhere), but supposedly the GLSL 3.30 features are almost finished. Who knows, that line of the document has been that way for a while, so maybe someone's already finished the necessary changes and not updated the document.