As it turns out, up until the end of 2005, WDDM (Windows Display Driver Model) was listed as a requirement of Vista, but a last stage decision mandated a downgrade of the program’s requirements to include Intel's outdated 910 and 915 graphics chipsets.
The decision of course pleased Intel as it saved them billions of dollars in potential lost revenue – the non compliant Intel i915 chipset was being deployed in a huge number of laptops back then – and a shift of business to AMD and Nvidia. On the other hand, HP was enraged given the significant investments it had made to upgrade to newer PC graphics technology based upon Microsoft's initially more demanding specs.
Ars Technica has summed up the contents of the emails in a recent article – quite an interesting read. The lawsuit, which began nearly a year and a half ago, was granted class-action status last February and is currently set to go to trial in April.