I just started this sock with Regia Nautica. It has looked a felt to me like a sport weight, but it's sold in the sock section of my LYS and I couldn't interpret the diagram on the ball band because it's teeny tiny and uses cm, not stitches or inches. Anyway, I just looked it up on Ravelry and it is a sport-weight!
That would explain why the first socks I knit with it are so thick!
I do like it. The colors are gorgeous and the tweediness is very rustic and pretty. It is splitty, though, I would not use it on anything that would require much frogging.
The pattern is Spring Forward from Knitty. It's an odd choice, now that I know I'm knitting a winter sock, but it is really pretty.
Teaser: I have an FO for Friday made out of the same yarn.