Here's a short video that describes the course for Ironman Tahoe: Ironman Lake Tahoe course
The first minute or so is pretty cheesy but the real info starts at 1:18.
Update: The run course has since been changed to be two loops, each going only half the distance. This means all the running will be on the Truckee River bike trail and the run course won't actually reach the lake. I guess they did this because it's much cheaper to run the race without closing parts of the main loop road but it's going to make for a very crowded bike path.