http://www.delta.com/skymiles/use_mi...x.jsp#upgrades
Delta Mileage Upgrade Awards
You can use your miles for a one-class upgrade on most Delta- and Northwest-operated flights. Mileage upgrade awards do not apply on all fares. To view the miles needed, see Delta Upgrade Awards.
http://www.delta.com/skymiles/use_mi...ards/index.jsp
If you have one of the qualifying fares Dilemma listed above you can upgrade with miles, whether you are elite/medallion or not, assuming there is upgrade space available.