From a 2007 article:

"One look at the all-new Madone is all it takes to see that Trek is a company reborn, or at least a company that has finally thrown the gloves off and ventured outside of its conservative box to reveal its true potential. Especially when compared to its somewhat staid predecessor, the new Madone displays a decidedly more aggressive profile and sleeker lines (including a first-for-Trek sloping top tube on its top-level race machine), more radically-shaped tube sections and lugs, and an eye-catching graphics package with heaps more 'pop' than what we're used to seeing from Trek."

From another article in the same year:

"Trek’s the last major manufacturer to jump on the sloping top tube bandwagon that uses marginally less material, thus saving a tiny bit of weight. In theory a smaller bike is more rigid and easier to handle."