Apparently this is a known problem -- see
this thread -- in one of calibre's support libraries. Since I don't remember seeing anything about it earlier, and now we've gotten three threads on it (one's over in the Sony section, waiting for the mods to move it here) within a couple of days, it's possible it's something that broke in 0.7.x, and may be in line for a fix. Hopefully one of the gooier gurus will stop in and provide an authoritative answer instead of my random flailing around trying to look helpful.