@Pencilmaster: That's just one theory though. Equally, the multiverse theory could state that everytime you go back in time, an alternate universe is created. Which is split in the timeline at the point you go back too, thus creating a whole new reality in which you are bound to. If you then travel back to the present (for you), things would have changed.
@Sausage: What's not to say that all time-travellers are sworn to an oath where they must not show themselves? And if some do, maybe there are "time police" that protect the natural flow of time by preventing these people from interrupting with history. Again, we can only speculate. But I do think time travel only belongs in science fiction.
@ICARAX: People have talked about this already to make teleportation. Essentially, you have a dematerialiser, that essentially kills you but saves all the data which is then sent to your target destination. And through a materialiser is reconstructed in your previous form down to the very nerve (if we get it spot on, theoretically all your memories would be intact). So, to achieve this, we'd have to achieve perfect cloning at a large scale (to be able to store all of the information of someone's body would be ridiculous and difficult to achieve).