"There Is No Half-Life 3"
No shit! I've known that for years!
Fact: There is no Half Life 3 until they announce Half Life 3.
Why is it so fucking hard for some people to get that through their thick skulls?
Valve gains NOTHING by keeping the game hidden until the last second. Look at how many people want HL3: I know that Valve would officially announce it when it gets the green light for full development.
Why? To build hype and let viral marketing do its job.
Announcing the game and NOT developing it would create the "cry wolf" scenario we've seen from the likes of Duke Nukem. If nothing else, it'd force everyone to reconsider their position with Valve a bit more rather than give in to the blind loyalty they currently have (and since the community is STILL trolling itself this hard after so many years, I'd wager they're still in the "blind loyalty" category).
Realistically, I'd wager that Valve's strategy for HL3 would be to use it as a means of showing how powerful/effective their new game engine can be, just like how HL2 was used to expand on the Source engine (from Counterstrike: Source).