Don't worry...it will be nothing like the impact of the marathon (which is more a traffic impact than park impact). The triathlon shouldn't have more than 1000-1500 athletes, compared to 25,000 for last year's marathon and 1/2 marathon. There will likely be some traffic congestion around Epcot, as at least part of a lane getting to the Epcot parking lot will be blocked off for bikes, and there will be runners running through Epcot. But the race is over at 1 pm...that's for even the less fast competitors. The super speedy ones will be done between 9 and 10 am.
Also, you're probably going to be missing a part of the participants. Again, another thing (though trivial in the greater scheme of things) affected by Hurricane Katrina. I would imagine this event was drawing athletes primarily from the SE...and the NO/Gulf Coast refugees have probably lost their bikes, as well as will have other things to be worrying about.