Space Boundaries

[dafrick]

There may be times when, in a level, we don't want the players to be able to get to far away from where we want them to be.

You need to think of a way of creating some kind of 3D-boundary, that can be added to a level in XML. How can (almost) arbitrarily shaped boundaries be created (and represented in XML)? And how can you make the boundary feel as part of the game world instead of just an unwelcome obstacle. (e.g. a forcefield, whose strength is inversely proportional to the distance from the boundary)