David Spencer of Valve Software was recently asked why there wasn't an Episode 3 trailer at the tail-end of Half-Life 2: Episode 2, and his answer was that the difference between it and the eventual finished product (as was the case between Ep.1 and Ep.2) would be vast and, therefore, rubbish.

"If you look at the Episode Two trailer that we shipped with Episode One there's some pretty radical difference between what you see there and see in finished game.

"That's really an artefact of making a trailer for a product that's still in heavy production. You just don't know where you're going to end up," he said, reports Action Trip.

And on Episode 3, Spencer added that Valve is going to push the boat out:

"We're going to try and do something pretty ambitious for that project", he added. "As long as there isn't a similar development time as between Episodes 1 and 2, we'll start looking forward right away."

As long as there isn't a similar development time as between Episodes 1 and 2, we'll start looking forward right away.