Always wondered about that too....it could also be in reference to Glory who was also called The Beast, so until we get Joss, tie him to a chair and make him answer all our questions we may never know, heh.
When it comes to shows like 24 and The Shield I try not to read to much into the preview because just when they seem to show you to much you end up getting a whole lot more.
a few possible ideas, though they're pretty run of the mill stuff so they're possible but I'm expecting something bigger. 1. someone from a big case Jack worked in the past that got away. 2. Someone Jack trained with in the millitary, either in the capacity of them being in training together or someone that Jack trained....<shrug> That's all I could come up with so far..
How about the fact that the rest of the cast had to read about SMG deciding not to return to Buffy after season 7 instead of telling them herself...I'm not one to rush to judge but that's pretty screwed up to do, especially when you've been working closely to people for seven years.