Naahh, I can't see it myself, but according too an article on Shacknews and I quote..

Some of the first bits of info on the upcoming Half-Life 2 expansion pack have made their way out. Via PCGamer UK edition we get this blurb. Thanks to Gonnas for getting us more accurate information

But as you will see quoting seems to be the flavour of the day, the following is a PCGamer quote..

Of course we're interested in telling more about the universe we have created and therefore we will continue making sequels to the game. At the moment we're working on a game where you play as Alyx and her well-known robot dog - Dog... I can't tell more at the moment

oh come on!, there are more quote's and this one is from Doug Lombardi himself...allegedly

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

Anyway the full article is over on www.shacknews.com.