Actually, Valve has started work on EP3. They have realsed some conceptual imagery, as which gives you a hint into the outcome of the game.
http://www.eurogamer.net/articles/va...three-progress
http://www.shacknews.com/onearticle.x/53545
Just these two things alone tell me that EP3 is well under production, and also that the Borealis and more combine advisors (The floaty things) will be involved.
Also, interestingly enough, there is a mention of the Borealis being an Aperture Science's Project. Perhaps Portal will be given some form of a mention?
Either way, I'm sure it is well underway, and I place my estimates for release at around March/April this year (2010)