i can't tell if you're describing a dent, or a hole, or slit in the tube. post up a picture of it so we know what we're dealing with.
if it is a dent of the dimensions you listed, short of having the tube replaced (which you can't really do with aluminum because of it's brittle nature, and the heat treatment that cannondale uses), there's nothing you can really do to fully "repair" it. you can fill in the dent with something, but it's not going to repair it in the structural sense. it'll look nice, but as far as the tube and it's ability to deal with load, nothing can really be done.
that being said, it sounds like the dent you're describing isn't that bad, and i would just ride the damn thing and not worry about it. the top tube of a frame doesn't bear as much load as the down tube and seat tubes and most of the time, a small dent is inconsequential to the structural integrity of the frame. if there are creases around where the dent is, i would sand the paint off around the dented area and check for cracks and keep an eye on it.