They will refund the unused portion of your fare if they cancelled or significantly delayed the flight.

From their Customer Service Commitment policy on their web site (see link below):
"Irregular Operations - Delays and Cancellations
Southwest always does its best to operate our flights as scheduled. Sometimes, events beyond our control or situations we could not anticipate prevent us from doing so. If, for any reason, your Southwest Airlines flight does not operate as scheduled, we will, at your request, refund the unused portion of your fare, or we will assist you by arranging to transport you to your destination on another Southwest flight with available seats. If you elect to take an alternate Southwest flight, we will not charge you any more money even if your ticket for the disrupted flight has usage limits or fare restrictions. Because Southwest offers high-frequency service in most of the markets we serve, we can usually accommodate our inconvenienced Customers within a reasonable amount of time."
Source(s):
http://www.southwest.com/about_swa/customer_service_commitment/csc.pdf

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