This reaffirms the notion that Derek and Cameron come from the same timeline. (Or does it?) But when and why did John send Derek back?
If Derek was sent back to 2006+, then he would have been wiped away when Cameron changed history in 1999, and wouldn't appear in the new timeline The Sarah Connor Chronicles takes place in--unless both John's send back Derek: the first so that Cameron can remember it, and the second so that Derek can actually reach the destination. The thing is, they'd have to send John for different reasons: the latter John would send Derek back knowing that Cameron would cause Sarah and John to jump forward in the future, and the former John would send Derek back because--the thing is, I can't figure out a coherent reason why John would send Derek back.
He'd have to have known that Cameron was going to change history, because presumably he wouldn't have remembered meeting Cameron as a teenager. (I toyed with the possibility of ignoring T3 and assuming that John did remember meeting Cameron, and having it all be one big causality loop--but no, Sarah's death by cancer screws up that theory.) So it'd be silly to send Derek back to any point in time after he sent Cameron. So he either didn't think things through--and I'd hate for the fanwank to only work by assuming the characters are stupid--or else he sent back Derek to before 1999. But that doesn't seem to work--surely Jessie would have noticed if he was suddenly a lot older? (We can assume that Derek jumped back and then jumped forward, but now we're getting to truly massive amounts of fanwank.)
Maybe there's some way John could send Derek and Cameron back at the same time (but to different temporal destinations) so that Derek would be protected from the effects of Cameron changing history? Call it the Stargate: Continuum school of time travel theory.
Ruling out that possibility, we're now back to Derek and Cameron being from separate timelines. But why would Cameron claim to know the reason John sent Derek back if they aren't from the same timeline? Assuming that John did, for whatever reason, try to send Derek back in Cameron's timeline, she could have assumed the same logic would carry over. This seems like a big assumption--if Derek's John knew Cameron as a teenager, that John might well reason very differently, but again, for the show to make any sense at all we do need to assume some degree of temporal inertia, that certain patterns (like Cameron's conversation with Jessie) keep happening over and over again in multiple timelines. Cameron would presumably be familiar with this phenomenon. But that explanation still doesn't seem totally sufficient to explain Cameron's utter certainty that Derek's mission was not to kill Andy Goode, but rather to wait for "us." There's no way she could be 100% positive that history didn't change in a salient way, is there?
The simplest explanation might be to simply assume Cameron is lying. But what could her motivation be? (Ooh, there's a fic there somewhere.)