While the open-source Intel Linux graphics driver already has support for at least 36 variants of Intel's forthcoming Haswell processors with faster integrated graphics, it turns out some were missing. The reserved PCI IDs weren't added and it turns out that some products are actually going to be using the IDs.
On a Haswell-based Intel Core i5-4258U processor it was discovered that the graphics weren't working on the latest Linux 3.9 kernel due to using one of these extra IDs, per this bug report. There's now a new driver commit by Intel's Chris Wilson to add in the reserved IDs for Haswell GT1, GT2, GT2+ parts.
Overall, if using the latest code on distributions like the brand new Ubuntu 13.04, the Intel Haswell Linux graphics support appears like it should be in good shape. However, there's been support for features like HiZ and other Mesa/DRM improvements that didn't land for Ubuntu 13.04 and other early Q2'2013 distributions, which means some stable customers may be waiting until H2 until seeing updates pushed into their next distribution releases. When Haswell officially launches we will have extensive Linux performance benchmarks at Phoronix.