It's certainly a surprising move on the surface but Colbert is a huge fan of Tolkein's work - he reportedly speaks Elvish, which is a whole other degree of dedication - and visited the set last year.
Is Jackson going to take advantage of Colbert's ability to actually speak Elvish? Or will he purely be stashed int he background of a shot? Time will have to tell because Colbert - at least so far - isn't.