Half Life 3 - Announced at Gamescom?

Makes sense. :)

Well, yes, but despite usually moving people from one project to another when developing their games, Valve does sometimes keep developers working on a single project, as they did in the case of 'Ricochet 2'. So releasing several games or updates in quick succession doesn't always exclude a new launch where a team has been kept consistent.

I guess you're right. If they do announce it it'll probably go down like this:

"A mystery conference is announced for Gamescon. People attending the event suspect it might be HL3 (of course). They attend the conference hoping for the best. The room is lit at first, but becomes pitch black once everyone’s seated. It’s silent for a minute or two, everybody is nervous… All of a sudden, Hazardous Environments blasts out of the speakers in the room and the crowd goes insane like at the announcement of the new Zelda game at E3 2004. The most epic trailer in game history starts playing…"