While patches have been floating around for a while and there was a separate branch to Mesa, Intel Open-Source Technology Center developers have now mainlined HiZ support for Haswell.
When Sandy Bridge launched, it took months for HiZ support, which is a performance-boosting feature. The timing with Ivy Bridge for HiZ was better while now with the forthcoming Haswell, there's pre-launch support.
The Hierarchical Z support can allow for performance gains by checking pixels against the z-buffer early on so that unneeded pixels aren't rendered. Intel's HiZ support can lead to impressive performance improvements for OpenGL on Linux, similar to Radeon's HyperZ.
While we aren't able to publish Intel Haswell benchmarks yet on Phoronix, the reported Haswell HiZ support reportedly introduces no Piglit OpenGL regressions and for one of the sample workloads referenced by Intel's Chad Versace is able to improve the GLBenchmark performance by 18.5%.
The commit where the merge to master happened was intel/hsw: Enable hiz (v2). Sadly, this HiZ support will land in Mesa 9.2/10.0, which won't arrive as a stable release until after Intel has introduced their Haswell processors. Those users of non-rolling-release Linux desktop distributions will likely be waiting until their H2'2013 updates to see this performance-enhancing feature.