This Makes My Head Spin
I was watching The Sarah Connor Chronicles tonight, and I started trying to trace lines of causality. (I know, bad idea.)
Humor me, and tell me if this makes sense:
The premise of the show is that John and Sarah Connor are trying to stop the creation of SkyNet, in order to stop the war between SkyNet (and its cyborg army) and humanity.
It was precisely that war which resulted, in the original movie, in Kyle Reese being sent back in time to stop the future governor of Kalifornia from killing Sarah Connor.
Kyle Reese, of course, is John Connor's father.
But...if John and Sarah Connor prevent the creation of SkyNet, and therefore the war never happens, then Kyle Reese would never have to be sent back in time, and John Connor would have never been born.
If John Connor was never born, then he and his mother wouldn't prevent the creation of SkyNet, and thus the war would happen...resulting in Kyle Reese being sent back in time to save Sarah and father John...leading to John and Sarah stopping the creation of SkyNet...and so on, and so on.
The only way I can see to prevent this mobius strip of causality is if John and Sarah fail to prevent the creation of SkyNet.