This OpenGL ES 3.0 branch of Mesa currently has "pre-alpha quality" support for OpenGL ES 3.0 for Intel HD hardware and it won't be merged until after the Mesa 8.1 release. However, Intel hopes to have beta OpenGL ES 3.0 support officially ready and in mainline Mesa for Q1'2013.
Within the GLES 3.0 branch right now there's some missing functionality like no support for the ETC2 texture compression (Ericsson Texture Compression), but Intel developers are working on it for their driver.
This information was confirmed by Ian Romanick on his blog and the OpenGL ES 3.0 code can be found via the GLES3 Mesa branch. Most of this GLES3 code is written by Ian; Nice job!
Too bad there's no secret OpenGL 4.x Mesa branch... For more details see OpenGL 4.3, OpenGL ES 3.0 Specifications Unveiled and OpenGL 4.3 Support Is A Ways Out In Mesa.