UPDATE 2ND NOVEMBER: Something did happen yesterday - a groan. Blizzard released a small Sombra hacking-themed ARG, another one, that amounted to a countdown, another one. Mercifully this new countdown points to BlizzCon this weekend where we expect Sombra to actually be unveiled and also maybe released.
Specifically the hacking ARG, which revolved around an attack on the website of fictional Overwatch company Lumérico, eventually revealed these translated words from Sombra (via Reddit):
"Good job, folks. I would not have done it without your help. Anyway, I got the resources needed for my next hit - will love. Wait [to] hear from me in the coming days."
The slight silver-lining was a new Day of the Dead spray appearing in Overwatch overnight.
ORIGINAL STORY 1ST NOVEMBER: A piece of Sombra artwork has been found on Blizzard's official gear/shop/store website, signed by an artist who works on the Overwatch team. It's since been removed but the picture has been preserved on Imgur via Reddit.
The image depicts Sombra standing on a giant omnic robot's hands, presumably manipulating it with the purple tendrils of light streaming from her high-tech gloved hands. Could this be something to do with her ultimate ability?
This depiction of Sombra matches almost exactly the depiction of her in a leaked image from a few weeks ago, suggesting that image was legit.
The artist named in the picture, John Polidora, is a senior concept artist on Blizzard's Overwatch team. Prints of his Sombra artwork are presumably going to be sold in conjunction with the character's reveal/release.
We expect that reveal to happen this Friday evening (UK time) during the opening ceremony of BlizzCon 2016. Sombra may also be released onto the Public Test Realm not long after - perhaps immediately.
There could also be something revealed today, according to the Sombra ARG that never ends. Today coincides with the Day of the Dead festival, which is linked with Sombra's Mexican origin and various clues found relating to the game's Mexican Dorado map. Blizzard tends to favour Tuesdays when adding things to Overwatch, too - but so close to BlizzCon why not wait? We'll have to wait and see!