Well, it's finally official. All signs were pointing to Microsoft pushing the Windows 10 Mobile update to older Windows Phone 8.1 devices today, March 17, 2016, and it's happening. In a Windows blog post, Microsoft makes it real.
Here's the gist:
Today, we’re pleased to begin the roll-out of Windows 10 Mobile to select Windows Phone 8.1 devices. There are a lot of great new features in Windows 10 Mobile, like Continuum, Windows Hello and Cortana.
Consistent with our approach to the Windows 10 PC upgrade, we are focused on delivering a high quality experience for our customers. The Windows 10 Upgrade Advisor app can be downloaded from the Windows Store to see if the upgrade is available for your device.
Microsoft points you to this page for a list of devices running, but we're not seeing anything there. We'll keep looking for an official list of supported devices, but in the meantime you can run the UpdateAdvisor app on your Windows Phone device and see if you're eligible.
Update: Microsoft provided a better link to supported devices, and so here is information on which devices are supported for both the Windows Insider program and the production release.