According to Game Informer magazine, Valve are not planning to ship Half-Life 2: Episode 3 in 2010. Episode 2 was released as part of the 'Orange Box' pack in 2007, alongside Team Fortress 2 and Portal, and word on Episode 3 has been very limited since then, despite the original plan being for all three episodes to be released in just eighteen months following the release of Half-Life 2 itself in November 2004. The magazine goes on to speculate that it's possible that Episode 3 may have even been cancelled and replaced with a Half-Life 3 running on a next-gen version of the Source Engine.
At the same time that this disappointing news has been circulating, an artificial reality game has appeared via the latest Steam updates hinting that further developments on the Portal 2 front may be imminent. The Portal team have been working on the sequel since shortly after the original game was released, despite the departure of original designer Kim Swift a few months ago to work for another company, and it sounds like some sort of announcement or preview is now on the way. A Portal 2 could certainly ease the waiting pains for the release of Episode 3.