You're in our world now is the slogan of EverQuest, the popular MMORPG released by Verant. It really has two connotations. The first and more simple meaning (and the one that they probably intended) is that by playing EverQuest you are removed from the real world and set in an alternate reality, hence you have moved from your world to their world.
The second and perhaps less friendly view of this statement is that you have in essense lost control of the environment by playing this game. This is not to say that anyone ever has full or perhaps any control of his or her environment. However, Verant is well known for making vast changes to the game (i.e. nerfing the game) without, seemingly, any input from the vast array of paying customers it has. For this reason this slogan appears to have more of a negative meaning than a positive one. Perhaps a lesson for any well-intentioned game development company in the future.