Like someone said earlier, there were only a tiny handful of mods for Deus Ex the first and most of them weren't any good anyway.
BUT if this will be the case with Thief 3 there will be blood in the streets. That game has a HUGE SP mod community and not releasing an SDK will be a betrayal of the worst kind. Hell, if Looking Glass released Dromed and ShockEd, neither of which can be considered stable by any stretch of the imagination, I'm sure modders would find a way to work with the Thief 3. In fact they only released ShockEd a little while ago and they said something liek "Here it is. It's not useable but if you can make it work it's yours." And guess what? That's just what they did!
On edit: Can't pass this opportunity to comment again on just who their target audience seems to be once more. Who is it that can't play mods again? Which platforms? Somebody remind me.
This comment was edited on Oct 27, 15:49.