I'll add my S3 to the "glass breaking without earning it" crowd. I had the phone in my side pocket for half a day, during which time I was mostly either sitting and working on the computer, or getting up and doing mundane tasks around the house. Somehow, the screen broke right between the home button and the trim, just like the OP. Thankfully, it's a small break, and it does not seem to cause any problems. I'm just worried that the crack will spread.
That being said, after the crack appeared, I tried carrying my phone in my shirt pocket for a few days... and it popped out over hard flooring or concrete three or four times (and I felt appropriately foolish for this). However, the only damage it sustained was a little scratching on the back case; no problems with the glass.
Considering my experience and what I've read in this thread, I think there are two important things to keep in mind with the Galaxy S3. 1: Flex is your enemy. The phone should be able to take some moderate falls (or even 5 to 6 foot falls onto concrete), but do not let it flex! 2: I think there may have been a bad batch of glass. Assuming that the phone doesn't fall in a manner that strongly flexes the glass, I don't think this screen should break from standard use falls.
Everyone else who has had your screen break when it should not have, please post! I don't know how much effect a thread on Droid Forums could have, but it could only help for us all to speak out.