I had this as an E3 prediction, but it seems that they wanted to get the announcement out of the way...
QuoteThe news most of us have been waiting for has finally been confirmed: Valve is readying the first expansion pack for the unanimously acclaimed Half-Life 2. Provisionally titled 'Aftermath', the new adventures of Gordon Freeman and his able sidekick Alyx is tentatively scheduled for a 'summer' release, according to PC Gamer UK, which has - incredibly - scooped the world with its cover feature on the hotly anticipated game in its May issue, out today.
http://www.eurogamer.net/article.php?article_id=58579
...hopefully it will have less airboat, and more monsters from hell.