Finally heard back from Del Rey, after six months.
The word is no. Bottom line: "the fundamental setup just doesn't hold up in terms of suspense." I don't agree, obviously. But the sting is lessened somewhat by comments like "I'm loath to pass on getting a chance to publish a talented writer like David."
I am not as crushed by this rejection as I was by the Tor one, because I hadn't been led to expect an acceptance and I wasn't as emotionally invested in it. My agent is querying other publishers right now, and I'll be talking with people at World Fantasy Con.
Onward and upward.