There is some information and many pointers on this page. The common idea is that Firewire allows DMA: the plugged device can read and write physical memory at will (this allows fast data transfer without clogging the CPU; it is very useful for hard disks and fast network interfaces). For Firewire, some mitigation can be achieved by removing or nerfing the Firewire support on the OS (most Firewire controller require the OS to do a bit of initialization job, and we remove that part -- but there could exist Firewire controllers which will accept device-initiated DMA even if the OS does nothing, so it is hard to know if actual protection is achieved that way).
PCMCIA/PC Card/CardBus/ExpressCard are successive names for a peripheral standard which is, basically, a PCI (or PCI Express) bus with safe hotplug. DMA is consubstantial to PCI, so there is little defense against an attacker who has physical access to such a port.
Generally speaking, if the attacker can physically access the machine, you are doomed.