While I haven't heard any accounts of garments being used as a weapon in this way, I've sat through many Fast and Testimony Meetings* where my co-religionists have provided first hand accounts of the sacred underwear's protective ability. A few of these stories have even made it onto the internet tubes.
I was in a major traffic accident the night before President Kimball died in 1985. I was in a subcompact car doing 60MPH when I was faced with a 1/2 ton truck that had gone out of control and was broadside in front of me. I do not know how fast he was going, but we were on a major highway and he was travelling towards me.
I should have died in the accident, according to the police. I was suspected of having massive internal injuries, a broken back, as well as my right leg hanging like a piece of meat below the knee and serious facial injuries. Anyway, when one of the ward members came to visit me in the hospital a week later, he pointed out that all my injuries(other than severe bruising)were above and below the garment line.
annegb tells her husband's story:
My husband went out to cut wood on the mountain and he accidentally cut through his pants with the saw. Well, I mean, he thinks the garments saved his leg because the pants have a big wide cut in the thigh part and his garments weren't cut. So, there is something there. I think it would be very hard to cut through a pair of levis with a chain saw and not through your leg.
Of course the big question is whether garments can protect you against a Rovian whisper campaign on the eve of Super Tuesday.
* Fast and Testimony Meetings are monthly meetings where churchmembers "bear their testimony" about the truthfulness of the gospel. One of my favorite testimonies of all time was one given by Sister "Smith" in which she thanked the Lord for allowing her to have another child (her tenth) long after she'd given up hope of ever giving birth again. She gave this testimony a couple of months after her son, "Bob," and I snuck into her and her husband's bedroom and stole the big box of condoms they kept hidden under their bed.
Note: That was fun. I think I'll do a weekly "What Mitt Believes." What should I start out with? Baptizing the dead? God lives on a planet near Kolob? Indians are Jews who ride tapirs? Joseph Smith and the magical translation stones? Why God embraced integration in 1978? Jesus: using dad's prestige to get his own universe? Why don't the prophets talk about the sunpeople and the moonpeople anymore? How Spartan-style wresting helps you determine whether you're speaking to a demon or an angel? Other suggestions?