This is the hottest tshirt trend right now in my opinion.
This is a technique that I got from Camille who posted it on her blog youngflames.
You can do it on ANY type of tshirt as I have found out, even a regular garden variety non stretch tshirt. Camille says that it works best on knit or stretch type tshirts but just experiment.
Be aware it is time consuming but VERY easy and effective!!!!
Fist cut the hem off your shirt or put a cut in to the material where ever you want shredding so that you have a raw edge.
Next get a sharp pointy object (like a quick unpick or a sewing needle or a pin) and 'grab' at the raw edge of the material with it until you get hold of a small loop of thread.
Once you have got hold of that loop, with your left hand index finger and thumb, hold the material on either side of where you're shredding taught, and with your right hand holding the quick unpick, pull the loop of thread away from the material to unravel it. Pushing the quick unpick against the surface you are working on is a good idea...
Once you get the section of shredding going it gets easier and you can just drag and scratch the quick unpick over the raw edge of material and the threads just unravel! Et Voila you now have couture!