OK Ben, good point. A shapeshifter is probably not scinetifically possible, but perhaps smoky is controlled or IS a supercomputer who can access the brain (studies first, brainwashes next) and then can implant the images to it's subject. Like Dave, only Hurley could see him. Only Eko could see Yemi, but that doesn't specifically mean it's all in his head, it just manifests itself there for the purposes of now interacting with the subject. Eko didn't 'play along' with what smoky had had in mind, so it sent it's iron filament self to (form of, an elephant) crush it's subject who would not obey!! make sense... a little ?