If you are traveling on Delta and want to know if your flight is oversold, send me an e-mail with your flight number and date of travel. I have a relative who works for Delta and she has given me her employee number and work date, with this information I can access Delta's VRU line.
The advantage of this system vs looking at a seat map on the internet, is that it gives the actual number when a flight is oversold.
For example a flight I checked for this Sunday afternoon has 3 seats left in First, but was -19 in coach. The system also gives the number of seats which they intend to sell, in this case Delta was selling 1 more coach seat which would make coach -20!
The VRU line also gives inventory for ASA and Comair.