
The first XQD cards from Sony were announced today and will go on sale in February. The 16GB XQD card is expected to sell for $130 U.S. while a 32GB version will sell for $230.
The primary benefit of the new type of memory card is speed. Using PCI Express as a data transfer interface, the new cards offer read speed from 500 MBps, write speeds from 125 MBps (expected to go up to 500 MBps in future generations), and potential future storage capabilities beyond 2 TiB.
The format was first announced in November 2010 by SanDisk, Sony and Nikon, and was immediately picked up by the CompactFlash Association for development. The final specification has approved in early December 2011. The cards are not backward compatible with CompactFlash or CFast cards.
Practically speaking it means the new XQD cards can record up to 100 frames of shooting in the new Nikon D4's raw photo format.
The following image (courtesy of Nikon) shows the dual CompactFlash and XQD slots of Nikon's D4 SLR. The XQD slot on the left with the card sticking out is significantly smaller than the CompactFlash slot.

Discuss the new XQD Flash memory format in Digtal Home's Portable Electronics Forum