This is funny...we were there in May of 05 and there was a half Ironman triathalon going on that week, and now we are going to be there the 17-23rd and apparently there is another race!
I'm not sure if the September race is the same as the May one, but I can say the May race caused very few problems for us. The only thing that was a bit rough was the transportation around FW (where we were staying). They didn't have boat service to MK during the swim and they had to reroute some of the buses to avoid runners/bikers. They did have signs up before hand and during the event to tell you how to get around. I think a lot of the issues we had were due to the fact that we were in FW...people at other resorts probably didn't even notice.
As far as impact on the parks, I really didn't notice a difference as far as attendance. The crowds were pretty light the whole week we were there. We overheard some of the athletes talking with their families about the race, so we know they were there, but it wasn't enough people to make the parks seem more crowded.
My only problem will be keeping my marathon running hubby from wanting to join the racers.