With Apple's announcement of the MacBook Pro they also obsoleted a popular notebook technology - the PC Card slot (formerly known as PCMCIA).
ExpressCard/34 is a blazing fast replacement for the venerable PC Card slot found on many notebooks, including several generations of PowerBook. ExpressCard is a serial interface delivering between 480 Mbytes/sec and 2.5 Gbit/sec/direction of bi-directional throughput, depending on the interface (USB 2.0 or PCI Express) while a CardBus PC Card is a parallel interface capable of only 33-132 Mbytes/sec.
The ExpressCard/34 is 34mm wide whereas the PC Card is 54mm wide, so current PC Cards won't fit in the new slot and aren't compatible. People that rely on PC Card media readers will be forced to switch to an external USB reader or direct cable connection until EC/34 media readers from the likes of Belkin and Buffalo (like the MCR-4/EX) begin shipping next month.
A bigger issue are high speed EVDO cards because they won't fit in the ExpressCard/34 slot either. There currently isn't a workaround to use an EVDO PC Card with ExpressCard because there aren't Mac OS X drivers for the U132 adapter. A Verizon representative confirms that they're hard at work on an EVDO ExpressCard and that it should ship around the time the MBP does. EVDOInfo.com is tracking developments on EVDO cards for the new ExpressCard/34 slot.
The decision to drop PCMCIA in favor of ExpressCard/34 probably wasn't a hard one for Apple. It's incredibly fast because it can access the full bandwidth of the USB or PCI Express bus and it's only a matter of time before the peripheral and accessory vendors jump on the ExpressCard bandwagon.
An interesting read is Dell's technology white paper "Industry Transition from PC Card to ExpressCard Technology" - it was published in September 2003.