I think ARMA is going to remain very niched among gamers because the vast majority of ARMA II sales at this point are to gain access to the free DayZ mod.
The ARMA brand recognition is unarguably much higher than ever before as a result of DayZ and that will probably help ARMA III's sales to some degree. Also assuming there is at least a small portion of DayZ players that tried out AND enjoyed ARMA II itself, it should be good for the franchise. However, many DayZ players give the ARMA II engine a shit-kicking over its unfriendly UI and mechanics. So it remains to be seen how it all plays out for ARMA III.
Once the DayZ standalone is out, I expect it to be in Steam's top 10 and ARMA II will completely drop out from it. Even this could be good for ARMA though, as it will keep Bohemia well funded.