IMDb has switched the "Untitled Steven Spielberg Project" to the 'filming' section of Meryl's filmography. It's listed as having started May 31 (yesterday), which aligns with original reports that shooting would be underway by May.
I'm still rather surprised that we have no information on other cast members at this point, but we know that info will be revealed eventually, and probably soon. And for those of you skeptics out there who still think that the film won't be ready in time for a 2017 release, we know that there are multiple films that have had even later start dates that not only made it in time, but were also good film. See Munich.
In other news, several sources are reporting that Amanda Seyfried has joined the cast of Mamma Mia: Here We Go Again! I didn't necessarily feel this needed its own post so I figured I'd just tag it along with this one. Details on the shooting schedule of that film should be forthcoming.