I would say its probably going to run on the ARMA 3 engine, one reason is because ARMA 3 was suppose to go into a public community alpha around mid-late 2012 which makes since for a September release of the standalone version of DayZ. I don't really see them taking the ARMA 2 engine for standalone and then updating later to the ARMA 3 engine. So if there is a standalone version to come out in September I could only think rocket would want RV4.