Just my theory, but the next half life episode will begin to merge with the story line of Portal. Valve is working the story line of Portal until it finally catches up with half-live, as well as making sure that the game engine is fully mature to handle both the Half Life game and the Portal game. This will allow the portal gun to work in Half Life. Just a guess on my part.
Notice their "summer" dlc that was supposed to be released "mid-september" is now being released in october.... and yet for the most part people are still completely content. Why? Because people appreciate the fact that valve actually has enough respect for their own product to finish it before it gets released, not to mention enough respect for their customers to make it free. None of that "release a half-finished game and then sell the rest of it a month later for another $10" bullshit. Thanks Valve