Setting aside the difference between SPORT-tourers and sport-TOURERS, I am not sure that there is a hard and fast definition. Tom, you recall I had a DL650 as well, and rode the crap out of it, and I would agree with you that it doesn't qualify as an ST. It's fun up to highway speeds, but to ride it really aggressively (140+ in the twisties, GPS top speed confirmed at 170 kph fully loaded with a tailwind on a closed course, officer) you have to wring its' neck.
I think that any sport-tourer, regardless of where the emphasis is, has to be capable of being ridden really really quickly really effortlessly. I don't think displacement is a factor - 600 SS would qualify.
At the heart of a ST is an engine designed to go fast and a chassis designed to corner well. An ST is about performance on a long haul. Some people can dress up a Busa to make it work, others overload a poor FZ1, still others buy a more purpose-built machine like a Concourse. *shrug* in the end, who cares as long as you're riding far and fast.
The ST is about a personal set of choices and compromises between SPORT and TOURING, and there's a different definition for each of us.