For now only SD and SDHC cards. People expect (rumor is) that XC could be supported in future firmware update for the HERO2 though.
However, you might also need to buy a bunch of batteries (or battery back-pacs, double the battery life). For example, shooting 720@60fps (my preference) or 1080p, my 16GB class 6 card fills right about the time the battery runs down. Also, I think the battery will die before the card fills with timelapse photos.
So, in my case, 1 battery per 16GB is needed.
Until very recently, GOPRO suggested that a16 GB minimum class 4 card (now class 10 for 0.5 second timelapse) is the best bang for the buck.
I saw one vid thru google search, though it states SDXC cards would work, but no further details. Gopro "tech assist" confirmed it might work but format is not guarantee.
Also some reviews on UHS-I cards are slower than Extreme previous versions.