Uhh, Valve takes a lot of time to make AAA title's, if you just look up the history of HL and HL2 on youtube or something, you'll most likely be able to find interviews and stuff in which Gabe himself says that when they were making half life 1, they worked on it for years, showed it off, then completely scrapped the game and started over from the beginning because they didn't think it was good enough anymore, the same thing happened with half-life 2, and I personally believe the same thing is happening with HL3, I think they maybe got super far into development, saw that it was already being surpassed by other games in terms of graphics and animation and other such things, and probably scrapped it and started over with a new engine in mind.

P.S. He also mentioned that the reason they don't like showing anything off unless they are absolutely finished, is because things change, and they might end up changing their minds, so what you'll see in the end could be completely different.

That they haven't said anything should be telling enough. Besides its not like the fanbase didn't give them the finger when HL2 was leaked.

They have been working on it. Though it seems to have been put on hold and/or scrapped and re-developed at certain points.

Valve is well aware fans want HL3, and since they haven't said they aren't developing it, its a safe bet they are working on it.

After what happened with the HL2 leak, they aren't taking any chances.

Gabe was asked a few years back on one of the gamertrailers shows if they are working on the next Half Life and he said yeah we're definitely working on it.

I guess no one watched that episode except me because it was never reported as news and I've seen no one talk about it.

No I'm not going to post a link, but you'll have to look through the GTTV archive to find it if it's there. I think it was mostly about Left 4 Dead 2 or Portal 2.

Also, consider that it's likely going to be on a new engine and HL games are long. It takes longer to make long FPS games these days which is why almost no one does it (other than Deus Ex HR). But I think Valve is stepping up to the plate on this one.