Game Informer got the word late last week from Valve that Half-Life 2 Episode 1 would be released on both Steam and Retail shelves on May 31st for a price of $19.95US. Episode 1 starts off where exactly where Half-Life 2 left off, and will answer some burning questions fans have had since the game was released.
Game Informer also had an interview with Gabe Newell and Doug Lombardi while at GDC last week, about not only Episode 1, but Episode 2, Steam, and several other subjects. According to the interview, Gabe Newell was able to run through Episode 1 in about 7 hours. SiN was touched on, along with other games being distributed on the Steam platform.
Link: GDC Valve Interview