I assume that the Source 2013 update will have access to linked portal doors. I think this could be awesome for dream sequences or hallucinations, maybe even a house scare.
It seems like it would be great for horror games because of the sense of disorientation it gives.
Video example.
https://www.youtube.com/watch?v=eHqhLzKqQlk
Wasn't sure how to put the video directly here.