I just checked my flight tomorrow, #255, PHL-PHX. As of yesterday (after the Silver window), seatcounter.com showed F5 and there were at least 10 seats in coach. In fact, I hadn't noticed much change in the overall load for that flight all week. Today, the entire flight is sold out.

I've seen this happen before: a flight suddenly sells out the day before a flight. This seems rather odd, but I suppose there is an explanation. Any takers?