I run Panaracer Pasela 700x32s on my Cannondale all of the time. I used to do 25s and 28s, but after going to 32s I decided I am not a racer and I like to be able to go off curbs and not pinch flat on a weekly basis so I have stuck with them for all my riding. They don't have much tread to speak of so they are smooth and great for commuting. I do ride some crushed limestone trails frequently (actually my fav riding surface, I love the sound) and they are ideal for such situations. That would be dry trails, when things get moist it is a different story, you may want some knobbies for that. Plus the Paselas are 10 bucks a pop at Nashbar, so you really can't go wrong. Your Trek looks like it has plenty of room for such a large tire, but I did have to dremel off some inessential metal off my rear derailleur to prevent rubbing. I am really not anal about such things, but if you are take a close look at your frame to make sure you won't rub against your down tubes.
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