Just came across this - http://makearmanotwar.com/
Opens for entries in 2014 - I'm really excited to see what comes from it.
Welcome to Bohemia Interactives 500,000 Arma 3 content creation contest. Building upon the Arma series strong heritage in modding, were challenging people to deliver their best work yet. With cash prizes ranging from 20,000 to 200,000, spread across four different categories, this is your opportunity to move onto the radar of hundreds of thousands of Arma 3 players.