Video Copilots Andrew Kramer has uploaded an excellent indepth tutorial outlining the after effects techniques used to create a portal effect as seen in their Demon Cam movie.. The tutorial covers ::

Chroma Keying, Screen equalizing, Spill Suppression
Building the Portal using included Smoke Clip
Colour Correction, Glow Effects & Optical Flares
Accurate depth revealing effect using ZPass from a 3D program

They are also giving away the project files.

Link to the website