Not sure if you get to see them in Atlanta, but Southwest is running tv ads right now poking fun at other airlines' nickel-and-diming strategies - the ad shows a guy having to stick quarters in a slot to open the overhead bin, put down his window shade, etc. And yet, with fuel costs where they are, how tempting it must be for Southwest to think about charging for assigned seating (even though they don't use them for assigned seating, the signs with row and seat numbers are in place on all their planes) or even just "auctioning" off A & B boarding passes (e.g., you can pay $15 for an A pass at the time you buy, or $10 for a B, subject to availability). I would bet my house that these ideas have been proposed within Southwest before. I imagine that so far they have decided it would betray their brand too much. As costs rise, their position may change.
Not to go too far OT, but I see signs of Southwest getting away from being a strictly low-fare airline. I have a feeling that they feel they provide superior service (always on time, stellar safety record, friendly, convenient, etc.) and that maybe they can charge a bit of a premium for that. We are going to WDW in September. I booked an Airtran web fare for $59 each way (+$5 each way for picking seats). Southwest's lowest internet-only fare at the time: $129 each way! That was higher than even AA and UA (the other non-stop CHI-MCO options). Even now it is only down to $97 each way (Airtran is $92 but of course if you pay for your seat assignment it is $97). There are plenty of threads on here wondering why Southwest doesn't seem to be as cheap as in the past, and when are the low low fares coming out, etc. I fear that the good old days may be gone.
I haven't seen the ads, but SW's management has hinted that "Pay for A" may be on the way according to industry reporting (Don't know if it's accuate) So I don't think SW is COMPLETELY opposed to nickle and diming.. (and they day they implement, they will just change the adds! )
Flyertalk.com has had a whole thread on instances where SW is far from the "lowest" option. I like them not just because they are LOW, but because of their liberal change policy! I would probably pay a few dollars extra for that IF they flew in and out of ATL. (I work a lot in Birmingham so I do fly them some!)