I took a bump off of UAL's last flight MSP-DEN last night when a 737-500 showed up instead of the promised 737-300, and my boarding pass for NW 567 had the "S" on it.
But, I was able to stand by for the earlier NW 565 and when my BP was reissued for that flight, no "S". Hmmmm. Well, it's great to have such timely info here on FLYERTALK...
Daze