The mobile apps are definitely in the wild, my understanding (from some checks in the past and from the discussion in the other thread) was that it was just the 'normal' useragent for the phone with the addition of a WikipediaMobile identifier at the front (so that you could tell it was from the app). They aren't very common but I have seen some which I knew were app driven.
The reason this came up was that one of the discussions that came up in that other thread was to rewrite the user agent to be less unique which would make it harder for CUs to tell users apart (especially once they found out, which they often do and these discussions are on public lists). The compromise that Yuvi proposed was to send a 'special' user agent for checkuser so that it didn't lose the fingerprinting ability as much.
James