The target audience are the security professionals that show up to local OWASP meetings. These t-shirts will be given out to attendees as prizes for participation in games, projects, raffles, etc.
The membership of OWASP is generally male, 25-45, and corporate. These t-shirts, however, should be aimed at the less corporate among us and should be something that a 25-30 year old male hacker would want to wear out with their friends on the Lower East Side of Manhattan.
With that in mind, the t-shirt should minimize any preaching about the organization and rather just focus on creating a cool shirt that OWASP info happens to also be on. This might mean making use of sleeves or the behind-the-neck area to move elements directly related to OWASP away from the center of attention and including some kind of (probably geeky) illustrative design as the focus instead.
Please see the attached images for the owasp logo (ologo.gif) and another design company's take on OWASP t-shirts for part of a conference the organization had a few years ago (owasp-shirts.jpg). The logo is likely going to be hard to work with due to the colors and gradient that it uses, so I would encourage you to play with it as much as you want.