from http://arstechnica.com/news.ars/post/20060201-6098.html

Bill Gates has argued as early as 1997 that the operating system should only allow approved, digitally "signed" device drivers to be installed. Now it has been confirmed that the 64-bit version of Windows Vista will indeed have this restriction built-in. Even administrators will not be allowed to install unsigned kernel-mode drivers on 64-bit systems. Unsigned drivers that do not run in kernel mode can still be installed by administrators, but not by regular users.

To be signed, each driver must obtain and use a Publisher Identity Certificate (PIC) from Microsoft. Unlike the more lengthy and costly WHQL driver certification process, the PIC is given out for free and does not involve any quality checking of the driver code by Microsoft. It simply establishes the identity of the software publisher with a digital certificate. In addition to the free PIC, software publishers are required to purchase a VeriSign Class 3 Commercial Software Publisher Certificate, which costs US$500 for a one-year license.