From the Trek site:
Integrated Speed Sensor

An industry first. Our new Speed Trap™ technology allows you to put a 2.4 GHz wireless sensor directly into the fork leg itself. The result is a clean look that preserves aerodynamics and a sensor that installs in seconds. Say goodbye to the zip ties. Speed Trap technology is built into Bontrager's Madone-specific Race and Race Lite forks, and is found on Trek Road models 1.5 though Madone 5.1.
Note: Fork includes integrated pocket, but Speedtrap sensor unit must be purchased separately.

So I would conclude that:

Looks like you have to install the sensor into the fork for it to work. The sensor is not there when you buy the bike, just the ablity to integrate it is.