Southwest has this ongoing ad campaign on TV where they talk all about their no hidden fees model.  They promote the fact that they do not charge check-in, phone reservation or window/aisle seat fees.  I cannot vouch for any of these except the last.

Southwest is correct when they say that they do not charge for window/aisle seats.  They cannot do this since they don't assign seats!  That said, IMO experience, they do something very close that in my opinion is only marginally different from charging for premium seats.

Follow up:

Let me explain, I recently traveled on Southwest to Las Vegas and as part of it, I had a very inexpensive fair.  It was an exclusive web fare and was the cheapest offered by Southwest.  I felt fortunate to get something so much cheaper than the friends I met there, but I believe that this cheap airfare impacted my seat assignment.

The new boarding process from Southwest revolves around assigning numbers supposably based on when you check-in.  The idea is that the sooner you check-in, the the lower your number and you can check in 24 hours in advance.  The numbers are divided into three categories with 1-60 in Section A, 61 - 120, Section B, 121 - 180 Section C.

My experience flying on the cheap fare was telling.  On the way out, I checked in 5 minutes after 24 hours.  (e.g 23 hours 55 minutes before the flight was to depart.)  The crazy thing is that with that, I received number A 59 which says that 59 people checking in before me!  It seems very unlikely that 59 people were able to check in during the first 5 minutes.  I had a similar experience on the way back and ended with with B1.

My believe is that Southwest chooses the number boarding number based on fare price.  Thus a cheaper fare will never get access to the low boarding numbers of the more expensive fares.  This very different from the previous approach which was based solely on letter (no number) and the letter WAS based only on when you checked in.

In short, if you want one of the best seats with Southwest, you need to a more expensive fare.  Cheaper fares are penalized with higher seat numbers which brings me back to the subject.  This analysis suggests then that there are actually fees to get the best seats.  They may not be extra add-on fees, but the core point remains that you must pay more for better seats.

In short, I believe that the "No Hidden Fees" state is misleading.  They neglect to mention that your fare directly impacts your seating options.  In my opinion, this is a hidden fe, but of course, they are not highlighting this fact.