Sadly this isn't happening, Valve rejected the itch.io app and released a mealy-mouthed statement explaining why they took it down from Greenlight.
Here's the full statement released by a Valve representative. (Taken from this tweet by Leaf.)
Hm, well guess that answers that. I know the answer may seem like a canned corporate spiel, but when your a company of that size formality matters. Ultimately though, I'd still call it a success, itch.io caught them off guard and gave them something to think about, and from the sounds of it they weren't entirely put off the idea. All thats left now is to keep making itch.io awesome and see what the future holds.