I would echo jogler's sentiment.
Not all Treks are equal.
The flex I saw was in the chainstay, quite noticeable on shyumu's Trek, but most definately not there on my Ridley.
Of course, being able to flex chainstays is a function of 1) your weight and 2) the amount of Watts you can put through the drivetrain.
I used to be able to flex the chainstays on my Claud Butler so much that the tyre rubbed against them. No chance of doing this with the Ridley. However the trade off is that the Ridley is quite a harsh ride. Effective at putting the power down, yes, but comfy over long rides, maybe less so...