3D Realms' Chris Hargrove made a post to their forums, talking about the scripting in their Unreal-engine title Duke Nukem Forever, and how it compares to that of Unreal's (thanks 3DPortal, by way of Unreal Universe). Here's an excerpt:

First off, DNF is not purely a TC of Unreal. We have made numerous tech changes inside the engine itself and will continue to do so. That being said, yes editing the script code for DNF will be a similar process to doing so with Unreal.

Everything game-logic related in Unreal is object based, and DNF is no different in that respect. Much of the script code work involved in making TCs and such will revolve around making subclasses to existing game logic classes, and in some cases modifying those original game logic classes directly (although this is a bit more dangerous and should be avoided where possible). Much of this process involves using UnrealEd. It won't be a "trivial" task to make a TC for DNF, but it won't be impossible, either. If you are planning on doing a lot of work in the code aspect of things, though, then get used to UnrealScript. We have made several alterations and additions to it, but the core language is still the same and will remain so.