CardBus is essentially PCI in a portable format. Most of the PCI signals are present, though some are multiplexed to save pin count.
All CardBus slots I've seen supported both 32-bit CardBus peripherals and JEIDA PCMCIA ones. This is doubly clever when you consider that PCMCIA is to ISA what CardBus is to PCI.
CardBus is designed so that you cannot accidentally insert a CardBus device into a PCMCIA slot. This is achieved not through the use of a metal strip- that's just for grounding- but by use of the two notches in the sides of the PCMCIA connector. CardBus devices only have a single notch, and so cannot be inserted into a PCMCIA slot.
The reason for the metal strip is simply that the PCI-like signals on the CardBus clock at much higher frequencies than the ISA-like signals of PCMCIA. This means that more shielding is required to prevent them from exceeding the limits placed by government on radio frequency noise emissions. By providing a means for the metal can of the card to be electrically bonded to the metal casing and ground planes of the computer, a Faraday cage is formed, preventing radio frequency interference from escaping.
The prime driver behind the adoption of CardBus was the move towards 100BASET ethernet networks; it's very, very hard to saturate a 100Mbit network using an ISA-like interface. Other factors included the relative scarcity of available ISA-compatible chips as the desktop computer industry moves over towards PCI, and the much greater scalability of the PCI bus.