The challenge, as usual, is to get to a build that can be used as the basis for the over the air upgrade for regular Windows Phone 8.1 users. We were expecting these upgrades to start happening just before Christmas (to the Lumia 640 etc.), but everything got delayed by the holidays. The new expected start date for this rollout is the latter half of January (2016).
Our editorial devices are updating as I write this (the usual spinning cogs for half an hour), comments welcome if you have particular observations. So far it's looking like the usual bug fixes and optimisations.
In theory, there should be news about this build in 'The Insiders Hub', but that's not working for me on editorial devices right now, so....
Note that if you don't see this update (10586.63) then you probably need to go back into the 'Windows Insiders' application and approve access to the 'fast ring' again. This may involve signing in again and also restarting the phone.
The delays for the Windows 10 Mobile updates to every day users on Windows Phone 8.1 is unfortunate, but if you've been living with W10M day to day like me then you'll know that there are lots of little annoyances which would be frustrating showstoppers to anyone not as patient and as err.... enthusiastic about the OS as us. There's no point in rolling all this out until it's 99.9% ready. 95% just isn't good enough.