Planet Half-Life asked Valve's Erik Johnson to clear up a few Half-Life 2 rumour that have been going about:
"Rumor: A Half-Life 2 SDK (Software Development Kit, what mod makers use to help make mods) will be released before the game is released.
Erik says: "Yes, we will be releasing an SDK for the Source engine for MOD developers before the game is shipped."
Considering the SDK for the original Half-Life wasn't ready until months after the game's release, this is great news!
Rumor: The Day of Defeat team is already working with Source.
Erik says: "The Day of Defeat team is currently working on the follow up release to Day of Defeat 1.0, which will be version 1.1. They are not yet working with the Source engine."
Erik also confirmed that there will be a Half-Life 2 Mod Expo, and more information on that event will be released soon."