I got the first rewrite condition in a post from jdMorgan:
D'oh! As soon as I saw the name I remembered that there is one situation where the host is empty-- and I should know this perfectly well because it's part of the usual domain-name-canonicalization redirect. "Exactly such-and-such or exactly nothing."
Bingo. That's one of the six alternatives I'd gone blank on :)
Sooner or later you'll have to figure out what to do with people who are using 1.0, though. Some proxies, possibly some satellite connections. I looked it up recently; humans with 1.0 do still exist, though they're not common.