from Faq
So, interested to understand what's new for 2010? Were you not able to use on same day prior?
Here is the explanation from delta.com... looking for the interpretation from all of you experienced Medallions....
In addition to the existing ability for customers to request an upgrade in advance, the new Systemwide Upgrade Certificates will be valid for use on the day of departure. This means if a First or Business Class seat is available on your Delta flight on the day of departure you will be able to request the upgrade.
Platinum Medallions will be able to select 4 Systemwide Upgrade Certificates as a Choice Benefit. Diamond Medallions will be able to select 6 Systemwide Upgrade Certificates as a Choice Benefit.
As with the existing Platinum Systemwide Upgrades, the new Systemwide Upgrade Certificates will be good for travel within or between the 50 United States, Bermuda, Canada, the Caribbean and Mexico on any Delta published fare booked in Y, B, M, H, Q or K class or Northwest published fares booked in Y, B, M, H, Q or V class. For all other international travel, upgrades are valid on any published fare in Y, B or M class.