[Sloth]

Seems to me that it might not let the bullet penetrate the fabric, but what's to keep the bullet from pushing the fabric and itself into the skin, leaving something of a wound? I know a hit to kevlar vest can leave a whopping big bruise. And this sounds a lot more flexible.

That's what I was thinking. "Bulletproof" is a very misleading term if the loose fabric merely doesn't puncture, but gets pushed X inches into your torso by the bullet... though I guess extracting the bullet will be easy for the ER staff ("Nurse, yank on his shirt tail.")

The only exception I can think of is if the fabric instantaneously stiffens when stress is applied, sort of like how your selt belt locks if you jerk forward quickly. If you've ever played with a thick corn starch & water solution, it changes physical properties when pressure is applied... I've always wondered what would happen if you shot that stuff with a bullet. I hypothesize that it would harden quickly enough to shatter into fragments, then the fragments would turn back to liquid.

[Othniel]

Wasn't Iron Man's armor something like this? Flexible like cloth until Tony Stark passed current through it, then it was tough as....well...iron. Wow. Will the shirt come with a solar powered tie?

[Redcloak]

The only exception I can think of is if the fabric instantaneously stiffens when stress is applied, sort of like how your selt belt locks if you jerk forward quickly.

Wasn't MIT working on just such a fabric a few years ago? The idea was that sensors in the garment would detect a sudden shock and apply a voltage to align the fibers into a hardened matrix. The whole process would take place before the bullet has penetrated the fabric. As I recall, this was also a DARPA project.