If you click here on FirstUSA's website (Southwest Airlines card):
http://www.firstusa.com/cgi-bin/webc...hwest_airlines

and then click on the "Pricing and Terms" button, it says the following about a Cap:

"3. Southwest Airlines Rapid Rewards Visa cardmembers are eligible to earn up to 48,000 Reward Dollars per year on Southwest Airlines Visa Accounts."