I recently re-read the Goblet of Fire and was struck by Fred and George's amazing prediction skills. How was it that they managed to guess that Krum would grab the Snitch even though it was going to cost his team the game. While I know thet're inteligent such a deduction seemed beyond them, maybe someone managed to tip them off, or else they had a time turner.
Either way I think this incident involved time-travel, possibly because of the Dumbledore time traveller theory.

In my opinion some of the twin's products are going to be very important in the final book, however if they hadn't made the bet, been ripped off then told Harry they wouldn't have got the gold to start their business, therefore them placing that bet may have had a very large impact on events in the final book.

If not, you may also like to think about the twin's impeccable timing, always knowing how far to push something, when they need to back off, often being in exactly the right place at the right time, all explainable if they had a time turner (although I admit that if they knew the future they probably wouldn't have trusted the guy they made the bet with, can't remember his name)