Just because there isn't a seat ASSIGNMENT available at this time does in no way mean your don't have a confirmed reservation on the flight. Having worked for years and years in the travel industry I know all airlines hold back a certain number of seats for airport check-in, mileage status flyers, etc. This situation is NOT unique with Delta. I check my flights on-line frequently for better seats..or in some cases, a seat assignment. Usually something shows up.

Judy