I have had the same problem recently with my I9000 ... which was fine for almost 2 years and then recently got a ton of scratches and eventually cracked ... even the new replacement glass already had a scratch within 2 weeks ...
thought this glass was meant to be quite tough ... so what can actually cause scratches??
I am very careful not to put keys in the same pocket as the phone .. so def not keys ....
could be the gate remote though, but that is plastic !!
Sand in pockets could be a big killer. I've actually found that keys don't do anything to Gorilla Glass. Although I'm not a complete retard so I keep my new Galaxy S3 in a pouch.