Bicycle Mechanics - Is my fork caput?

I was riding my new Trek hybrid when I heard a wrenching sound inside my headset. There was no wobble in the headset but it looked uneven somehow. I opened it up shortly after and found the protion of metal that holds the nut that clamps my headset down was ripped. Eventually, the nut just pulled itself out when I tried to screw everything back together on top.
I'm assuming the fork needs to be replaced. I'm just wondering for warranty purposes and peace of mind, what generally causes that to happen? Second, does the entire fork need to be replaced?

I'm going to guess the cause was the nut being loose and damaging the threads over an extended period of time. However, it could be because the fork's steerer tube was cut to short (or a headset with a larger stack height than the original was installed) which would leave too few threads for the locknut to engage. Warrenty will depend on the age of the bike and whose fault it appears to be. Take it to the shop you bought it at, explain things nicely and hopefully you won't be charged for a replacement fork. The fork may be able to be saved if you use a headset with a smaller stack height, but don't count on it. Otherwise, the fork is done.

I just reread your post and realized you said the bike was NEW. This should definitely be under warrenty. Whoever put the headset on may have crossthreaded the locknut when they put it on.

I think you have a threadless style headset. A nut-with-wings is jammed inside the steerer tube. Some of these designs are better than others at keeping the nut in place. If you have a steel steerer tube, there should be no ill effects, you can replace the nut. Usually, you force the old nut out the bottom of the steerer , and jam the new one in from the top.

Im a bit worried about this happening on a new bike. I think there may be grounds for a fork replacement. In the UK, you use a phase like "Of merchantable quality" ( a quote from the consumer protection law) and the shop suddenly gets very attentive.

MichaelW - you may be right, I assumed a threaded headset from the description. If that is the case, no problem, easily repaired. Putting the star-fangled nut in is a bit of a pain and a lot of 'wrenches' at shops are inexperienced kids, so I could see this being installed wrong which would allow it to be pulled out when tightened down.

Thanks for the input... it is a threadless fork. The whole bike is about a month old. I'm heading over to the shop now for my scheduled appointment. It's only by coincidence that this happened two days before it's tune-up.

Well that was a bit strange. I brought my bike into the shop and showed the problem to my mechanic and he thought nothing of it. He was like, "Oh, well we'll just hammer a new" whatchamacallit "in there and make it stronger this time". I was relieved to say the least. I was expecting an "Oh my god what did you do?" with a worried look mixed with a hint of condemnation. A look I've seen more than enough times at other LBS's.
Ah well... thanks again for the input. I can now go on to worrying about other things like fixing this computer of mine.