Steam version only of this game on release.

http://www.bistudio.com/english/company/developers-blog/356-arma3-steam-dev-blog

From the blog:

1. So, when will you release Arma 3 then?
   1. "Soon" has become an overused word, but we are preparing all we need to unveil our plans right this moment.
2. Do I have to run the game through Steam?
   1. Yes, it will be required to run the game through Steam. We do not believe in always-online types of DRM however, so Offline mode will be supported.
3. Will I be able to buy a boxed copy of Arma 3?
   1. There will be boxed copies in most regions. These may either be Steam Codes in a retail package, or also contain DVD data (speeding up installation). You will still have to run and update through Steam the first time.
4. I don't want any of the benefits Steam brings me, so why should I care?
   1. There was a very real chance there would be no Arma 3 this year otherwise.

Really looking forward to this game. I just remembered it was coming out so I went to watched some videos no Youtube and now I'm considering buying the game right now to get into the Alpha.