Same here. I read December was out and when I went to check for it I realized I'm still on October. Really odd as I always seem to get the updates the day they're released for OTA???
On 7.0 from beta program and still on October security update as well...
Well, that's another theory shot to hell!
NBD91P for me which installed when I got home tonight
NBD91U is the latest Nexus 6 build, it contains the Dec security update.