I've successfully "stood-by" on a later flight for NW since same-day stand-bys are allowed. I went to the front counter, asked if I could get on a later flight and they went ahead and punched it up for me.
In other words, NWA does allow later flight stand-bys. If you don't check in for your flight at the required time, your reservation will be cancelled, but as long as you're standing by on the same day and there's availability on the flight you want, you've got that seat (although they may make you sweat it out at the gate).
More info on this can be found over on the NW boards.
aloha
p.s. Travelbuzz?
[This message has been edited by slippahs (edited Feb 08, 2004).]