I might be missing something here, but if one wanted to port something to Linux, they'd most probably have to recompile everything anyway and generally optimize it for the platform (i.e. use X-related libraries, switch to OpenGL if they have to, or whatever).. so why not textures? They might as well recompile textures, it shouldn't be too much of a hassle, right? I haven't ported a game to Linux before so I wouldn't know but that's what I would imagine the process to be like.
See above, it might be impossible if they don't have the source textures any more. Heck, they might not have game sources either.