<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Canarsie:
A and D fares are upgrades, not actual purchases of Business or First Class. The difference is that these upgrades are automatic when available on select routes.[/list]</font>
This I believe to be incorrect. Certainly for D fares it is. They are treated like F and C as far as MQM accrual. As far as availability, A or D must have free inventory on the flight you are going on.
However, there are the Y*DLUP fares (Y can be substituted for other economy inventories) that are upgrades, not purchased premium cabin. When buying those fares you are subject to either V or G availability and accrues MQM's equivalent to the inventory purchased.
Regarding the SAN question posed: Yes, there are A and D fares out of SAN in certain markets. However, they are not the super cheap A fares associated with AirTran routes. SAN also has a number of **DLUP** fares.
Delta appears to be still taking a match by market approach in pricing. Recently, United has started a very agressive price/availability strategy and for the first time I'm seeing extremely agressive fares even in markets not served by LCCs.