I thought the player/DM distinction was what separated Dragon and Dungeon already. How is this different?
Basically. In practice, Dungeon Magazine (at least in its 4E form) has mostly just been adventures, while all the coolest crunchy stuff was in Dragon -- even articles about monsters and villains, which are at best useless to players, and at worst a form of spoiler.Mainly in that stuff that only DMs could use like the Bestiaries and such will be taken over to Dungeon and in its place material oriented just at Players will be in Dragon.
I wonder if this is precipitating any kind of move to offer "split subscriptions" or something, where a player could subscribe only to Dragon Magazine, for example. DDI is already dirt cheap, but customizing the content of your subscription might draw additional subscribers.