Han Solo will have a cameo and will be played by Harrison Ford. The age difference will be explained by saying that in the Star Wars Universe once people reach a certain age, they start getting younger again. So, in episode III Han Solo is like the equivalent of Ben Kenobi's age in episode IV, but in episode IV he appears much younger. This will of course be more in line with Lucas' original vision.
The CG Yoda was accidentally deleted, but in act of quick thinking Frank Oz grabbed a Miss Piggy puppet and completed the rest of the scenes. Lucas has assured the press that no one will be able to tell the difference in the final film.
Anakin Skywalker really does die in episode III and General Grievous brain is put into the Darth Vader robot. In episode V when Darth tells Luke that he is his father, he is really just taunting him. The reason that Anakin's young ghost form appears at the end of episode VI is because when GG killed Anakin, his spirit haunted the Vader suit. When Luke burns the suit at the end of ROTJ, it frees Anakin's spirit so he can appear next to Ben and Yoda. This explains why he is the only young ghost, because that is when he died.
And the beat goes on...