For example, if someone invents a working method of time travel on January 1, 2004, then there will be lots and lots of time travelers going back to visit past years prior to 2004.
With all the changes they'd inevitably make to the past, sooner or later someone is going to end up making a change that will alter January 1, 2004 enough that time travel *doesn't* get invented. For example, they might accidentally interfere with the birth, life or ancestry of the inventor, or start events in motion that sent him into a career in painting rather than physics, or they might interfere with earlier inventions which the time machine used as parts, or... Any number of things.
So eventually the past will happen to be altered in a way that "uninvents" the time machine that would have been built on 1/1/04, and *poof*, no time travel after all.
Then in the new timeline, someone else will eventually invent the time machine on some other date -- until *it* gets "uninvented" by people twiddling with the past.
So Niven's conclusion is that even if time travel into the past is possible, it'll keep preventing itself from being invented (for long, anyway).
On a similar note, I saw a trailer for an upcoming film ("The Butterfly Effect") which seems to be based on a somewhat related idea. A guy figures out how to travel into the past by sheer concentration, and he tries to prevent his dead girlfriend's death in the past. But every time he goes back and makes a change, the "ripples" of unintended consequences keep making the big picture even worse... It looks like a good film.
There are a lot of theories on time travel that gets around temporal paradox issues. None of them are complete, but they are based on our current understanding of quantum physics.
One theory is that traveling back in time would result in the traveler ending up in a parallel universe where everything appears to be the past, but is really the past in another universe that is very close to the one he left and when he returned, it would be back to his original universe. That way nothing he did in "the past" would effect his future.
Another theory is hard to get your mind around. That part of quantum theory is that cause and effect would get turned on it's head regarding time travel. According to some theorists, the laws of cause and effect would become reversed, so that your actions in the future would influence the your actions in the past. I've been trying to make sense of that one for a while. The problem is that we are corporial in nature. Our understanding of the universe is that there is a beginning, a middle and an end, in that order. For us to understand how space and time operate on a grand scale is like trying to explain three dimentional space to someone who only exists in a two dimensional world. All they can understand is right and left and would not be able to understand the concept of up and down.