While we haven't gotten any official confirmation from either Microsoft or At&t on when the Lumia 950 will be available.
Multiple sources have claimed a Nov. 20th release date (next week Friday).
What we can confirm is that it looks like the At&t Lumia 950 will ship with the Windows 10 Mobile Build 10586, as one was spotted running the RTM build recently.
It's important to note that the Build 10586 for the new upcoming Lumia 950 and 950XL is different from the Build 10586 Microsoft will be pushing out to Insiders next week, as the Lumia 950/950XL Build 10586 is optimized for those models. So it makes sense that the Lumia 950 are already pre-loaded with the RTM build.
The Build 10586 is already available for Windows 10 PCs, Tablets and even the Xbox One.
The mobile version was delayed because Microsoft was focusing on the Windows 10 PC version. But as Gabe Aul tweeted just yesterday, now that the Windows 10 update has begun rolling out, focus has shifted to the mobile version, he also confirmed it will take a few more days for it to be ready, so sometime next week.
In the meantime, let's look to Nov. 20th as the unconfirmed release date for the 950.