All of the conditions for felting are there, friction with the inside of my shoes, heat and moisture from my feet. All of the signs of felting are there, too, loss of stitch definition, completely opaque fabric, loss of stretch, and retention of shape.
I wasn't expecting it, but it makes sense, and I'm actually kind of happy about it. The stress points of the sock are less likely to wear holes through now, as felted fabric is stronger. They're also molding themselves perfectly to the shape of my foot.
Has anyone else experienced this?