As the title says, the DLC appears to have completely broken the authoring tools. Can anyone else confirm this? I've already sent an email to Gabe.

Yes Rufus, all of us are having the same issue. There is an open thread in fact about that before yours:

http://forums.thinking.withportals.com/mapping-help/authoring-tools-not-working-t4734.html?start=0

It seems that with the DLC, it came also a new update for the authoring tools that has screwed it up.

I guess Valve knows that and must be working on it... Let's see if it remains by this evening...