I finally got round to watching the amazing Half-Life 2 Portal Video and it reminded me of the locales and boundaries that I implemented in MASSIVE-3 (which were themselves based on the Locales in MERL's SPLINE).
MASSIVE-3 represented the virtual world as a partially replicated scene graph. Locales were frames of reference in the scene graph and boundaries were links between locales. Transforms on boundaries defined the locales relative positions, scales, rotations and rendering effects, allowing CC-TV boundaries to tiny versions of distant locales, boundaries to shadow versions of locales, torroidal universes of locales joined by boundaries which linked back on themselves and many of the effects which Richard Bartle suggests are impossible in a graphical virtual world.
We then added the concept of temporal transforms across boundaries which contolled the replay of events recorded in the target locale allowing links to locales in the past or to slow motion versions of current locales (we didn't manage links to the future, unfortunately).
Locales and Boundaries in MASSIVE-3 were lots of fun to play with and the Half Life video shows that they're even more fun with gravity and guns.