I'm still not sure about my cover letter. I've read a lot of conflicting advice online, so I would up briefly describing the piece ("murder mystery set on a space station where certain implications of faster-than-light travel represent a critical plot point"), mentioned that this is my first fictional work but that I've had scholarly work published in a scientific field. Etc, etc. This leads me to the last reason I sent to S&SF first: GVG has said that he reads the cover letters last. :)
Having spent the last few days psyching myself up to firmly believing that it will be accepted, I'm now trying to convince myself that it is certain to be rejected, and indeed added a folder to my filing cabinet for "Rejection Slips". Seemed like the prudent thing to do.