For users of released hardware still running old Linux kernels, the experience can be buggy if it's from early on in the hardware enablement timeline. Intel has been working on Haswell and Valley View support in the public spotlight for months with early support already in the Linux kernel, but the quality isn't yet pristine.
Intel will now be hiding this early VLV / HSW support -- along with future hardware -- behind a i915_preliminary_hw_support module parameter. When booting the Linux kernel with its primitive future hardware support, i915.i915_preliminary_hw_support=1 will need to be set in order for the Intel DRM driver to active on the hardware. This variable is being disabled by default.
The patch with this change, which Daniel Vetter has already pulled into his tree, can be found on the mailing list.