Quote from MinotauroDunno, but I'd rather work as an environment artist, seems a lot nicer than just planning levels all the day
We get to do both
We get a lot of trust put into us. We document our own levels from start to finish (coming up with most of the ideas ourselves, aside of basic plot points and specific scenes, and the construction of those is still entirely up to us). Using Radiant is another plus here - with patch meshes, we can also build most of what we want to build without touching Lightwave. We do model terrain manually, which I feel gives much stronger control than any heightmap terrain does, but it can be a little tedious at first.
We do use modeled props, most of which are concepted up, built, and textured in the art department. However, some of our designers also do their own textures and map objects - we can build a quick low or high poly model in LW, clean it up, and let the artists take over. Or we can build the normal maps for a texture, render it out, plop a grey swatch over it, and leave it to the artists to paint. Generally we just let them do the prop work, as they have it scheduled and we don't.
Scripters curently do all event scripting, though a couple designers can do some scripting as well. They're also doing stuff like monster, item, and trigger placement right now, too, as a time consideration (that's stuff they can easily do while we work on construction for another map). Eventually we'll have more of a hand in this and learn basic scripting all-around. They'll still be doing the really crazy scripted stuff, though, as most of us designers don't have much of a background in programming. I do hope to learn a lot about this and over time do more and more of my own scripting (but not enough they bump me out of design, heh), and I'd recommend you guys looking for jobs in the future dabble in scripting for unreal or doom3 or something.
We pass maps around a lot, too. Sometimes it's because the person who originally made it is busy with something else, sometimes it's just to get another set of hands on it. We also steal from each other all the time.
Load up a map, take a section you like, adjust and re-texture, and poof! It's just another thing that helps efficiency as well as consistency within the game world.