Some recent web-based stories have had purported Half-Life 2 expansion info attributed to PC Gamer UK magazine. An update on Shacknews clarifies that the article in question is actually conjecture, and is identified as such in its own text. Further word on all this can be found on ComputerGames.Ro, where they quote Valve's Doug Lombardi:

"Despite the rumors, Valve has not announced any plans for an HL2 expansion."