Update: After having spoken to Marc, I can confirm that he is a Valve employee no longer. You can read his reasons in the now-authenticated email posted by Redditor TeddyWolf.
Original: Hark! What news do the winged monkeys of wild internet rumour bring? Could it be that Marc Laidlaw, sole writer on Half-Life and Half-Life 2, has left Valve? If a series of TeddyWolf's email screenshots (I know, I know) trying to crowbar Half-Life 3 information from Laidlaw are to be believed, the man himself is in retirement.
"I am no longer a full or part time Valve employee," the email reads, "no longer involved in day to day decisions or operations, no longer a spokesman for the company, no longer privy to most types of confidential information, no longer working on Valve games in any capacity."
I've bypassed the monkeys and contacted Laidlaw by email to try to find the truth in it, but if it is the case, "I am going to let this written statement stand as my complete comment on the matter" doesn't make it sound like he'll invite me round to talk it out. Or respond. But who can blame a man for wanting a quiet retirement?
Reasons given include being oldish and "feeling a need for a break from the collaborative chaos of game production". If it is a troll, their prose is lovely.
As to Half-Life 3, we know as much as we did before.