Believe it or not, Valve was working on a prequel to Portal, based on a design experiment initiative launched shortly after the release of The Orange Box back in the day. Known as F-Stop, which is a play on the word Aperture I imagine, it was due to become a full blown prequel to Portal.
In the end though, much like other Valve projects, it ended up fading away, never to see the light of day - that is, until now, as LunchHouse Software, with Valve's permission, are promising to release a series on YouTube which will show off F-Stop and explain it's history for the first time.
LunchHouse Software are actually taking up development of a new game based on the source code for F-STOP it seems, with the game now known as Exposure, with the blessing of Valve.
The mechanics are not based on speculation or hearsay. Instead, Exposure uses the original, official code from Valve's own F-STOP, or as it was properly named, Aperture Camera
We're not sure yet when these videos will be releasing, but there is going to be around 5-6 videos released in the series.
Stay tuned to GameFront for any further updates as we learn them.