The Secure Digital (SD) memory card standard is reportedly slated for a major revamp next year, one that will boost max data transfer speeds from the current 104MB/s to a whopping 300MB/s. To accomplish this, version 4.0 SD cards will see the data transfer scheme change from parallel to serial, using low-voltage differential signaling (LVDS), and gain a new dedicated pin for high-speed data transfers.
The new standard will maintain similar external card dimensions to ensure backward compatibility. As a result, newer cards will be able to rely on parallel transfers when used on older readers, but you'll need to update your hardware to make use of the higher speed. Keep in mind that the 300MB/s transfer rate is merely what the interface is capable of, so it remains to be seen if cards can actually push this number to the limit.