Google is pushing the Android 8.1 Oreo update with the December security patch today, after running through two developer preview versions of the software over the last couple of months, and only a short couple of weeks after the second developer preview was pushed out to users. This is the stable update of the Android 8.1 software but this isn't the over-the-air software hitting devices, rather Google has posted the OTA images and the Factory images for the Pixel and Nexus devices so users can flash the software to their devices if they feel the need to install the software manually, instead of waiting for the update to come through as an installable download over the network.
While there could be some new stuff that wasn't available or noticed during the last two developer previews of Android 8.1, it's also possible and likely that much of if not all of the changes with this stable software are changes that have been experienced during the dev previews over the past couple of months. Since this also includes the December security patch though users will finally get the fixes that were detailed in yesterday's publishing of the December security bulletin.
As to which devices have the images available for download it looks like the Pixel 2 and Pixel 2 XL, the Pixel and Pixel XL, the Nexus 6P and Nexus 5X, and the Pixel C tablet all have the latest Android 8.1 software available to flash in either version, both of which you can grab by hitting the buttons down below. With that the only device that doesn't have the new patch and stable Android 8.1 software but should be getting it is the Nexus Player as all the other compatible devices seem to have it available. If you're not one who likes to manually flash the software and you'd feel safer with installing an over-the-air update version, now that the OTA images and the Factory Images are live and available for download and flashing then the OTA updates being sent out to devices shouldn't be too far off. Taking that into consideration it could still be a few days or more before the Google does push those out over-the-air.