The prevailing theory seems to be when a room opens up because of a cancellation it shows as available immediately on line. So we can see it and if you call MS, they can see it.
But the waitlist for those open rooms is only run periodically against available rooms. So if you check on line after someone cancels but before the waitlist is compared to open rooms, you can book it.
I've done this a couple of times already. I was so surprised to get rooms at VGC only a couple of months out from my travel date, and there had been no rooms open for weeks while I kept checking, until one day they were. I posted on the board about how lucky I was, and two other members posted they had those days waitlisted and were amazed I had gotten them w/o being on the waitlist, and they didn't get their waitlist filled.
I really think this has been going on all along. Even today there's a new thread somewhere by a member who called MS about their waitlist and had it come through while they were on the phone. People have been reporting that happening for a long time. It when you call that MS actually does a manual check for availability, thus beating the waitlist system to the punch.
It's just now, with on line booking, that we can see a little behind the curtain and make an educated guess about this aspect of the waitlist system.