In the previous year where I had 100% upgrade success as a SE I was twice upgraded after already boarding and taking my coach seat.
When a First Class seat is not available, the system will place you on the airport priority standby list for upgrading at time of check-in (if available). You may check the status of your upgrade once you check in at the airport. The airport priority standby list is limited by the number of seats available in First Class on each aircraft. Once the list is full for a particular flight, no additional names will be added.
This process is automatic; Elite Services reservations agents cannot add names to the standby list.
http://webx25.nwa.com/cgi-bin/cgi_el...upgrades.shtml