Another question about portals...

By portals i mean the effect used in Valve's portal games and seen in the "portalizer" unity package. I have searched almost every related question on many forums, however none seem to offer the help I need. I have tried re-working the mirror reflection script found on the wiki, however I have a lot of problems when they are out of the camera's view, and it never seems to work. I feel like I have spent hours on this, if you know anywhere which details the logic and explanation of this effect such as camera matrices and stuff it would be great, or just a working block of code for me to look through.

By the way i'm actually not hoping to explicitly use it for potals, Im attempting to make a room similar to Doctor Who's Tardis which is bigger on the inside, kind of mimicking the effect of Non-Euclidean space. Im also looking for something more advanced than a simple camera with render-to-texture. Im also using the pro version.

Thanks in advance, meshocky