I actually don't think Anakin should appear as a force ghost at all for many reasons. First of all, it makes no sense that Sebastian Shaw (as he appears) would be the ghost. It makes a little more sense that Hayden Christensen would be the ghost, but it would be confusing to people who watch 4-6 and to Luke actually. Any way, I know Qui-Gon was able to discover the power beyond the grave, but for Kenobi and Yoda they needed to learn. It almost makes sense that they could have taught Anakin after he was dead, but then why didn't they teach the other dead Jedi? The biggest reason why I'm against it though is that if Anakin doesn't turn into a ghost, it makes his redemption much more bittersweet. When ROTJ ends, Vader's dead and you're actually sad. Then, oh wait a minute, ROTJ is a happy movie, now Vader's a ghost too! Vader was really evil at one time. I feel there needs to be a consequence for that. Having him be dead, and unable to find life after life, would be a much more poignant end to the saga.