The cards are great for storing pictures and other data. I've used a 2 gigabyte one quite a bit to transfer around files, and in particular relevance to WoW, large addon and configuration directories (the WTF folder in the root WoW directory). It's much faster to put the 150 megabyte of WoW configuration files onto the card and physically transfer it than to deal with a slow and shabby WiFi connection.
However with the introduction of this new technology an interesting prospect is raised: playing WoW off an SD card directly.
Panasonic hope to have a 64GB card available soon, and rumors are saying sometime around March. Of course the cards will be a couple hundred dollars most likely, but nonetheless when this happens I will be easily able to transfer my entire WoW directory onto the card and play it from there.
The speed of the data transfer will be such that it'll easily be able to handle WoW (which is itself based on five+ year old technology). And the added benefit of having to not update my laptop's game every time there's a patch or I change addons or configurations, which happens quite often in my job here at WoW Insider, will be more than worth the cost.
One could argue that I can already do this with a portable hard drive, and they'd be correct. But those, despite their usefulness, are quite bulky and I have a personal way of breaking them more often than not.
So it looks like this new SD card format will be something I'm picking up quite fast, if only to further ease the way I WoW.