I don't know exactly when Trek stopped US frame production, I was just using Sherlockian deductive reasoning. Waterloo/Whitewater were their only US production facilities, so since Whitewater shut down on '04, and your '07 model has a Made In USA sticker, ergo dipso facto unum (I don't really do Latin...), it was built in Waterloo.

Whitewater was a pretty incredible operation. Trek built lower-end bikes there price-competitively with Giant/Merida for quite a while. They brought in raw tubes for frames and giant long aluminum extrusions for rims, built their own frames, rolled their own rims. The mostly robotic rim-rolling/welding machine was very cool. The factory was close to a local technical high school, so they never had trouble hiring welder graduates.