[Pervasive games intertwine] "game play with peoples' everyday lives. Armed with their mobile phone or another portable device, participants can take part in treasure hunts for virtual objects in the real world, find new ways to explore and experience a city, arrange blind dates, act out role-playing scenarios or wage virtual battles with opponents via SMS messages."
The article has more ideas and details. I've envisioned this sort of virtual overlay game in a warehouse or theme park setting, but these guys want to play in the whole world. Sounds fun to me, but will there be enough players for long-term success?