I've been participating in triathlons for over thirteen years now. My complete lack of swimming speed keeps me in the middle of the pack, but it is a blast training with friends and participating in races. In 2004, 2005, 2006, 2007 and 2008, I completed half-Ironman distance races at Lake Conroe or near Austin. In June, 2006, I finished an Ironman triathlon in Coeur d'Alene, Idaho - what a long day! - 15 hr, 58 min. Maybe next time I can trim a couple hours off that time.
The sport of triathlon in the United States is governed by USAT (USA Triathlon). Their website (http://www.usatriathlon.org/) describes how to become a member, lists clubs, races, championships, and provides rankings for all members.
Another excellent website is Runner Triathlete News (http://www.runnertriathletenews.com/). Click on the "calendar" or "results" links on the sidebar to find events and detailed listings of race results in Texas, Oklahoma, Louisiana, Arkansas and New Mexico.
For several years I was advisor for the Texas A&M Triathlon Team, a student club. Check out their website at http://triathlon.tamu.edu. The team is very active in races in our region and at the collegiate national championships.