Valve has revealed that Half-life 2's upcoming expansion pack - Half-Life 2: Aftermath, will be renamed Half-Life 2: Episode 1 and will be the first in a series of episodic games.

It may be possible that Valve will be selling the episodes via Steam, like the upcoming SiN Episodes.
Valve promised a retail release of Aftermath back in May 2005. Though renaming and making HL 2 episodic hardly would delay things, one can't rule out postponement of the retail release date following this.
Never the less, HL 2 going episodic means that it would put in the element of 'what happens next' to keep the interest in the game alive.
Via: Gamespot