So, what's an ExpressCard/34? Can I use my PCMCIA (PCCARD) cards in it?
ExpressCard is a new standard from the folks who brought us the various PCMCIA standards (including CardBus). As implemented in the new Mac, it's physically smaller than PCMCIA (which will lead to at least one annoying issue that we'll talk about later) and sizes out at 75mm deep (into the machine), 5mm high, and 34mm wide. There is another form factor (the ExpressCard/54) that is 54mm wide, but it isn't in use on the Macintosh. Connectors for both of the form factors are the same, so there isn't any difference in what capabilities are available. The only difference is the size of the card.
From an I/O perspective, the ExpressCard replaces the CardBus I/O system with a combination of busses, USB 2.0 and PCI Express. This means that some cards (such as EVDO devices and other communications cards) are likely to look like the computer systems as USB devices, but use the ExpressCard form factor to fit inside of the computer. This has some distinct advantages, such as removing the need for bridge chips and logic used in todays CardBus cards to achieve this same effect, and the disadvantage of giving people the excuse of using a lazy USB implementation when a faster PCI Express interface might be an advantage. However, I'm figuring it's probably a net win for most.
Unfortunately, the down-side of the 34mm form factor is that you won't be doing things you've done with PCCARD slots, like inserting CompactFlash cards into your computer with an adaptor. Unfortunately, the 34mm slots won't accommodate the width necessary, so you'll probably be using a Firewire or USB adaptor for the time being (or just hooking your camera up).
It'll be interesting to see how quickly the standard is adopted. Since it has easy USB availability, it'll probably mean fast turn-around for things like EVDO modems, but there aren't any right now.