I called SVT about it today because I got another big spot on my seat yesterday. I put my hand on the driver's seat to reach over to the glove box, and when I lifted my hand there was a huge white spot, like someone poured bleach on the seat. It is definitely from stretching the fabric.
So... I called SVT about it, and the guy I spoke to had never heard anything about it. He put me on hold and asked a few others about it. Here's the story: it is from stretching the fabric; what happens is a powdery substance from the foam underneath comes out onto the fabric. I was told that a strong upholstery cleaner should clean the powder off. I will try to clean mine this weekend, and then let folks know how it worked.