Thanks Eric, got the same tip on twitter. I started to capture user agent to eVar with the help of processing rules. Of course you could do also script to s_code and save the info to eVar directly from the browser using "navigator.userAgent". Also learned that you can ask from ClientCare to get "clickstream data" where they could add also user agent data. That way you could get some historial data also, but this isn't important issue for us so maybe I'll pass this (since it's quite complicated when you have to send all the ftp details etc to ClientCare).
When I get enough data then I have to correlate the users of "Unknown Generic Windows RT Tablet" with the user agent data and hopefully then I get more detailed data about Surface users. I found this tip http://www.cheapstrat.com/cheappress/2013/pull-the-user-agent-strings/
But even with this user agent data I'm not 100% sure that are those all Surface users and/or can Surface users use other browser etc so this goes bit tricky. I guess we have to wait that Adobe Analytics recognizes Surface devices with 100% accuracy then we can be really sure.
Heh, this really wasn't imporant issue, but always want to learn new stuff and I hate if I don't understand something. :)