Most of the code to implement trackbacks is in place now. Mostly there just remains the CSS styling, creating the appropriate template variables, and setting up the appropriate options. It shouldn't be long before we have a working implementation of trackback (sending is finished, so far as I can tell; it's only receiving trackbacks that's yet to be completed).
The moderation system for trackbacks is built on the second generation comment model, so it will be fairly easy to keep on top of all new trackbacks and delete any inappropriate ones. Still, I think that displaying trackbacks may default to off.