The height of SD Card performance is about to increase with a new specification version 4.00 on the horizon, said to be ready by spring of next year.
The jump will go from a current maximum of 104MB/sec to a whopping 300MB/sec, but this of course means the hardware reading the card will need to be updated to take advantage of the much higher speeds as well.
The higher speed is accomplished by changing the data transfer scheme from parallel to serial using low-voltage differential signaling (LVDS). This also means the number of pins on the cards themselves and in the readers will increase from the current nine in order to get the speed increase. It's been said the new cards, despite having more pins, will be able to operate in current hardware, only the speeds will of course be limited to a max of 104MB/sec.