We're using Piwik and Google Analytics, and currently the only way we can determine whether a visitor used our wiki on a mobile device is via the browser model, since we don't have a mobile URL -- and even then it's not 100% accurate, since some people prefer to use the desktop version of the site instead of the mobile version.
I did a search and saw this article on X-Analytics. I'm not a MW expert so I'm not 100% clear on what X-A does, and from the code linked it seems to only track whether a user is in alpha or beta mode.
We're currently running MW 1.21 so the X-Analytics doesn't seem to be included in that MobileFrontend version. Hopefully we'll upgrade ASAP, but I wanted to see if this was a way to get better data on mobile usage of our wiki.
Thanks all!