According to the Terminator series, the logical corollary of this also holds: as well as being able to create an alternate present by changing the past, so an agent in a potential future can come to the present to change which potential future becomes the "real" one.
Then you have temporal incursions, which was Enterprise's name for kind of the same thing that SkyNet was doing: future agents going back in time to change the timeline for their benefit.
This perfectly sensible and logical diagram explains everything. Click for a full-size (readable) version.
Terminator's SkyNet is shown in the diagram as never being built in the "real" (most probable) future. A faint possibility exists that it could be built. If somehow that possible-future's SkyNet acquired enough "real-ness" to interfere with actual reality, it could attempt to bring itself into existence properly. In other words, a not-actually-real entity would be trying to make itself real. A demon trying to summon itself.
Should time travel be possible, then this would imply that everything that could ever possibly happen ever, does happen, in a sense. These alternates would exist as potential events, kind of how the position of an electron around an atom is a "cloud" of potential positions - in a sense it is in all positions at once but with the probabilities distributed in a particular way (in the case of the electron, the probability of finding it in any particular state is determined by its wavefunction, and observing the electron effectively converts the possible to the actual).
If you went back in time, you would be altering the probability of your own existence, just like poor old Marty McFly; if for instance you attempted to kill your own ancestor then you would presumably have to deal with some sort of probability barrier, since the closer you get to doing it the less probable it is that you exist to attempt it in the first place. This may in fact mean it would not be possible for you to do it; perhaps in the act you would become insubstantial as your probability of existence tended towards zero, and on withdrawing from the attempt you would solidify and become more real again.
In fact, even just transmitting information may end up coming out at the other (past) end completely randomised, since information transfer would affect the course of history, in turn affecting the probability of you being there to transmit the information - so the past end would see interference as the present end's probability of existence fluctuated.
So there you go, time travel paradoxes would never happen because the closer you get to changing something, the less effect you are able to have on past events. Kind of a temporal version of the uncertainty principle (time difference / effect rather than position / momentum).
This doesn't mean it's impossible for possible-future entities to attempt to bring themselves fully into existence - melodramatically, for horrible things outside existence to scratch at the walls of the world, looking for a way in. I don't see how something improbable can make itself more probable, but that doesn't mean it can't happen.
![[Tinfoil Hat]](http://www.ivarch.com/personal/blog/uploaded_images/emot-tinfoil.gif)
