Today, while thinking as I do while I listen to music, I realized something. The reason Microsoft implimented driver signing in Windows XP, actually.
Modifying your sound and video drivers is a way to get around any sort of DRM. You can slip some code into your sound card that will write to disk any audio passed to it, and thereby circumvent any DRM.
The WHQL certification has less to do with any quality control and more to do with making sure the drivers are "secure". Signing lets them certify that the drivers have been checked for "quality", and recommending signed drivers is a lead-up to requiring them. Or so they hope.
Drivers signed with a private key would be much harder to slip custom code into.