There are a growing number of websites that will pay you for t-shirt designs, the most popular of which is Threadless:

http://threadless.com/

You don't even need any design skills - just come up with a witty phrase, submit that, and they'll take care of the typography for you. If enough people vote for your idea/design, you get paid, plus you get the kudos of having people wear your t-shirt! What more could you want?