If you bring an outside world into this one, there are going to be repercussions: people will think about it, talk about it, live (and die) over and in it, and they're going to sell their items on ebay.

You can't preserve purity by seclusion.

You can, however, seek to balance not only the in-game mechanics, but also the cohesion between the two worlds.

I believe having multiple points of access to a game world is a good thing.

So far, the most standard way is the single-point-of-access interface: you log in to your character, and you log out.

Characters are the final step of identity in the game world, but there is also a step before that I believe has much potential:

The account.

(For example, a PvP player in Guild Wars might not vest much of his/her identity into individual characters, but the account itself which is a depository of unlocked skills and items - or parts of the world from expansions.)

The account/log in step decides what your perspective of the game world and your attitude towards the game world will be.

Players not only want to experience being characters in a game world; players want to experience the game world(s) as a whole!

In your game world, how can you provide players/users with additional access?