"A" fares in DL are discounted first class. They do count towards additional miles and MQMs (EQMs) @ a 1.5 rate. I use these all the time as they can be very similiarly priced to coach. They get you a guarenteed first class seat, more miles, more MQMs, and at a very reduced rate.

The only time they can cause you trouble is when you are on a reroute or equipment change. "A" fares will be trumped by "F" fares in the change process. If not enough seats are available in the front cabin, you will sit in back as the "A" fare is considered to be an "upgraded" coach fare.

Some useful sites in the Delta world: www.cwsi.net/delta.htm This has a great amount of information about the fare classes on Delta.

Also, if you want to see seats in both cabins on a Delta flight:
http://www.benedelman.org/travel/dlseats/