Oh, okay, that's what it was. Did some digging and it *is* the toolchain update, it's just optional on the server side right now as a pre-release so server operators can test and make sure their set-ups are working properly.
"We're working on an optional update for Team Fortress 2. The notes for the update are below. We are tentatively aiming for 4PM PDT (23:00 UTC).
This update will be making the toolchain beta branch live. This update is known to break numerous server mods, many of which have updates available. In order to reduce the pain with this transition, the update is optional for servers. However, the next mandatory TF update will necessarily end support for the pre-toolchain-update builds, so server administrators should seek to update as soon as possible.
See the "Updated system requirements for TF2 dedicated servers" thread for more information on the toolchain update."
-John Schoenick, Valve