Here is a definitive answer out of the newest Nov 01 edition of the FD booklet (just received; not included in last version of Apr 01...): (translated from German)

First class upgrade for RW at NW

NW introduced a new procedure for upgrades to F recently. RW members could get an upgrade into F on NW flights in the contiguos 48 US states and between the 48 states and Alaska, Mexico, the caribbean and Puerto Rico.

Upgrades on basis of a ticket at full coach fare (class S or Y):
The upgrade is confirmed at time of reservation.

Upgrades on basis of a discounted coach ticket:
upgrade gets confirmed 24 hours before departure, after midnight.
Upgrade gets confirmed automatically, if member travels alone or with a platinum elite companion.

If there is no free seat in F available at the above time, member gets automatically onto a priority waitlist for upgrade at checkin. Maximum number of entries onto this waitlist is the number of total seats in F for the flight. If this number is reached, no further entries are possible.


So this is quite a bit of change, isn't it?