There's been concept art leaked here and there, comments on 'episode 3' vs. 'half life 3', and little things like filenames and spare code and such in other valve games that appear to have come from HL3, but nothing really concrete.
That said, I don't see any reason to believe they're not working on it - not developing it just doesn't vibe with Valve's MO. But until there's some actual news regarding it, I'm not gonna get worked up every time there's some rumor about it.