Interesting! I'm reading HBP to my boys and "Spinner's End" was our chapter last night.

I think Snape didn't know. *shrug* I don't know if I can articulate exactly why I think that, beyond his hesitation while making the Vow and that scene at Slughorn's Christmas party. (But then I've never been able to figure out whodunit halfway through a mystery, either. I just don't read that way, or my brain doesn't work that way, or something. It's been a source of great frustration to me, honestly!)