NGNetwork has posted a two question Q&A with Epic's Tim Sweeney, talking about the next Unreal (1) patch (which he says will be released once UT hits the shelves) as well as this bit regarding mod compatibility between the demo and full versions of Unreal Tournament:

When the full Unreal Tournament ships, we expect UT demo mods will need a small amount of upgrading and modification, rather than having full binary compatibility. After UT ships and we start patching UT, we expect few mods to break. The game, AI, and server framework is really solid now. The kinds of things that are likely to break are mods that use the C++ headers (rather than pure UnrealScript), and mods which tie into implementation details that might change, like server browser minutiae.