Mark is probably the worst example to use since the SW trilogy were about the only major motion pictures he has appeared it. It is his first major appearence so everyone is going to see him as Luke, and he didn't get any big roles after that to shake it.

Ewan had roles before, between, and after. The PT was a very forgettable set of movies (though they shall haunt me forever), so while people might say about Ewan, "Hey, its Obi-Wan!" I would be inclined to say that is due to the popularity of the old films, which every one already knows Obi-Wan Kenobi from, and the over hype of the PT. If the PT had been free standing movies without any OT to fall back on, just three films from an unknown director (like the originals were), there would not have been a sequel to the first one, and therefore not three of them for one, and having been close to ten years now since TPM came out, I am sure it would have been forgotten. It would probably be remembered mostly by fans of the actors who look up their filmographies, see some odd obscure children's sci-fi film, and go out searching high and low at video rental stores for it, pop it in the old VHS player, have a good laugh, and return it nodding their head, "Oh, Ewan/Natalie/Liam, how did you ever get so hard up to sign on for that dog of a film..."