The Nexus Factory Images are out and the OTAs are scheduled to begin distribution sometime today. That means it's time for a huge AOSP code drop. Google is in the process of uploading Android 6.0 Marshmallow and its first official build MRA58K to the Android Open Source Project. Coincidentally, there are a handful of Android 5.1.1 updates (r19 through r24) making an appearance, no doubt to fulfill the monthly security update for October.
This is a major version release, so there's quite a bit of code to upload and that's going to take quite a while. Depending on how much has to be uploaded, it may not be done until late tonight or even tomorrow. The security updates shouldn't have a significant impact, but it may be a while before we can create a full changelog. We'll update this post when it looks like Google is finished with the upload.
And It's Done
It looks like the code drop is complete. By my estimate, the upload finished up somewhere between 7:00 PM and 8:30 PM PDT. We're building the changelog now, so it should be up shortly.