RC6 allows for increased power savings and improved performance by allowing the graphics processor to enter a lower power state. We knew Intel was planning for RC6 in Linux 3.2, but the commit was finally pulled today by Linus Torvalds.
It's this commit in the mainline kernel that lands the support. Earlier generations of Intel graphics hardware don't have RC6 enabled there since it's likely broken. The RC6 feature can also be manually toggled via the i915.i915_enable_rc6 kernel module parameter.
It's a bit interesting that Linus decided to pull this patch so late in the Linux 3.2 cycle when he's been cracking down on non-regression-fixes at this point in the cycle and RC6 has been known to be problematic in the past. Just recently Linus was ranting about another Linux DRM pull for Linux 3.2 that he ultimately rejected on the basis of not being a regression fix.