well, i've got a canoscan 1200 series and it's fantastic. weighs only about a pound and it's gotta be under an inch thick. of course, it cost 170 bucks. the 600 series is about half that price, and from what i've heard, it got good reviews...but i've never seen it myself. certainly , though, the canoscans are some of the slimmest, "portable" scanners around.
of course, "portable" really doesn't apply if you're scanning documents, because the scanner has to be at least as big as the documents!