Valve has revealed that the 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's unclear whether this announcement will affect the release date (previously announced as April 24th) or the distribution of the expansion pack.

Presumably Valve will be selling the episodes via Steam, similar to the upcoming SiN Episodes. Valve promised a retail release of Aftermath back in May 2005, but it's possible this may have changed in light of this announcement.