DVdoctor has a new article looking at PCI Express, how it works, and there's a bit of bus history in there too

PCI Express isn't just about increased bandwidth. It also offers better power management, including native hot-plug support. And, although the physical interface is not backwards compatible, the driver architecture is. So PCI products can rapidly be moved over to PCI Express with little rewriting of software needed to make them work.

DVdoctor.feature - PCI Express technology backgrounder