Taken from the latest edition of "Ask Ausellio" over at tvguide.com:
Question: Can you give me some Without a Trace scoop?
Ausiello: The show's crossover episode with CSI will kick off November sweeps. "It's about a serial killer that goes across the country," explains CSI boss Carol Mendelsohn. "We've had the story in our back pocket for a couple of years, and now that WAT is back on Thursday nights, it's perfect." Mendelsohn adds that the gimmick was "spawned out of the relationship between Anthony LaPaglia and Billy Petersen and [CSI producer/director] Danny Cannon; they've all wanted to work together."
[My notes: That's quite a crossover to attempt. Billy Petersen and Anthony LaPaglia on the same case...yikes!]
Also: Ausiello: Detective Mars is joining the cast [of Without A Trace]! No, not Kristen Bell. Or Enrico Colantoni, for that matter. Trace is adding a character named Detective Mars, a male in his mid-to-late thirties who specializes in human trafficking. Of all the names they could've used, did they have to go with that one? That's just cruel. Almost as cruel as me leading off this answer with, "Detective Mars is joining the cast!" OMG, this just in: Detective Mars is being played by none other than Spike himself, James frakkin' Marsters! He just announced it on his website!