With a kernel patch done by Intel's Ben Widawsky to their DRM driver, a correction was made for Sandy Bridge and newer (Ivy Bridge, Haswell, etc) graphics cores to take care of a kernel driver bug that has been around for several months. With this patch that's now found in drm-intel-nightly and may make its way to stable kernel series, overclocking should work. Without the patch, Intel Turbo support is broken under Linux.
With a kernel bearing that Intel Turbo GPU support, overclocking of the Intel GPU core should be possible from the BIOS/UEFI of many enthusiast-grade Sandy Bridge and Ivy Bridge motherboards. For the motherboards that allow manipulating the GPU core's frequency and voltage, with Sandy Bridge and particularly Ivy Bridge the GPU can be pushed a whole lot faster.
More details on the Intel Linux GPU overclocking can be found on the blog of Intel's Daniel Vetter. Some Intel Linux GPU overclocking benchmark results will be published on Phoronix soon.
Unfortunately, this work isn't about any initiatives to make GPU overclocking easy from the Linux desktop with any user-space programs for manipulating Intel Turbo, overclocking, or other features.