FreeBSD currently supports PCI Express hotplug devices in head. This page will describe the issues with hotplug support, underlying infrastructure needed to support hotplugging, and device drivers etc. that need to be written.
PCI hotplug device drivers
PCI Express standard hotplug
Supported since r299142.
Compaq
IBM
Dell
PCI-SIG
PCI infrastructure
Most of this is now handled via the NEW_PCIB driver changes.