It looks like that T-Mobile U.S. update advisory was right on the money. A few hours after T-Mo updated its support site with word of a security update for the Nexus 4, we're seeing signs that an update to build JWR66Y (from the earlier JWR66V) is pushing out to the Nexus 4 and (GSM) Galaxy Nexus. Posters on the XDA forums have said that they've received a small OTA update to the new version, which still identifies itself as Android 4.3.
It's been speculated that the new version addresses Android security vulnerabilities which were recently involved in a high-profile Bitcoin heist.
So far we're not seeing any signs that the Nexus 10 or either Nexus 7s have been updated, but we'd expect the update to be extended to all Nexus devices sooner rather than later. Let us know in the comments if you've already updated your phone this morning.