Mouselook in space, without a 'down' or 'up'

I'm planning to make a game that allows you to fly around in outer space, and it's important for the 'space' feel that there is no 'down' or 'up'. For example, in a standard FPS on the ground, when you look all the way up, your character can't look up any further. On top of that, if you move the mouse to the side, your character spins around but is still aiming at the same point, directly above him.

I need to make a camera that, even if you're looking straight 'up' from the original orientation, moving the mouse to the left makes your aim move to the left exactly the same way as if you were turning left while aiming at the horizon in a standard FPS. This would result in your view of the "world" (outer space) becoming tilted, but that's the point. Also, there shouldn't be anything stopping the camera from tilting 'up' so far that it flips over altogether.

Can anyone give me advice on how to implement such a camera?