I've also been thinking about why Valve is not planning on releasing Ep3 until late 2007. It is very possible that they are leaving some development time to update Half-Life 2 with new graphic effects, primarily HDR. Or, maybe they have another game-mode, or game (TF2, CSS2, etc) they want to focus on after Ep2...or hell maybe they just plan to make Ep3 longer and need more dev time.
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Currently playing GW Factions, Day of Defeat Source and HL2:Episode 1.