Both times at the gate with DL. In fact, the first time I went up to the agent and was simply requesting to get an exit row seat. She looked at her computer and said "I can do better. Since you are medallion, I'll put you on the standby list for an upgrade." When I questioned her since I was on a CO ticket, she said that doesn't matter. So the next time I flew DL on a CO ticket, I just requested to be added to the list and the gate agent added me no questions. Don't know if I was lucky both times or not. First time was May2004 and the other time October2004.

Below is from Delta's website. The question is what does "certain Delta-designated codeshare flights" mean? I'm assuming this has to be other carrier's codeshare tickets on DL metal, but the wording is not clear.

Complimentary Upgrades
Your Medallion® membership now entitles you to enjoy unlimited complimentary upgrades, subject to availability, on most published coach fares on Delta and certain Delta-designated codeshare flights in North America, within or between the United States (excluding Hawaii), Bermuda, Canada, the Caribbean, and Mexico.

These complimentary upgrades are not valid for travel to/from Hawaii, and may not be used on Song®, Delta Shuttle®, or Delta Connection® carriers. For applicability on other Delta-designated codeshare flights, contact a Delta representative. Delta limits seats available for use with complimentary upgrades and may not make seats available on all flights.