the chain is like two weeks old, since I had to replace the one that this phenomenon destroyed about a month ago. It has about 300-400km on it. The rings are as old as the bike, which I got in October of 07. I don't ride all that much except during the summer, so total mileage on the bike shouldn't be anywhere close to wearing-through-chainring levels. I'm guessing it's the result of some sort of damage, but nothing's happened to the bike in the last month or so that I can think of and the problem has come seemingly out of the blue.