The 'Southwest effect' is described as the two things that happen when Southwest starts flying into a new city. Airfares drop across the board as established carriers match Southwest's typically lower fares, and air travel increases in response.
Here's an interesting example of the other side of the Southwest effect. For reasons best known to itself and to no-one else, Southwest only publishes its schedules (and therefore fares) a little way into the future, unlike most airlines that will accept reservations up to 11 months in advance (and even further in advance for group type bookings).
So guess what happens on the particular day when Southwest's future bookings stop being available? Yes - airfares typically rise on other carriers. Here's a wonderful example of this in chart form, courtesy of farecompare.com - at present, you can book with Southwest through until 6 March 2009. Look at the leap in fares on United that occurs on 7 March - from an average of about $385 up to an average of about $440.
If you're flying anywhere in the US, plainly it is best not to book further in advance than Southwest has its schedules published for.