Repost of what I put in the "Hail Galactus.." thread. There's already too many threads with the same subject matter and we're a year away...
I still say that a physical Galactus IS NOT NECESSARY to the film, if and when he appears. Nobody has confirmed the Galactus storyline for the second film yet anyway.
Any way they do it - costume, makeup, CGI, whatever will disappoint somebody. They could do it completely with a voice and physical manifestations of his will, such as the converter appearing, the carrier nimbus, storm effects, etc.
Does anyone remember actually seeing God/Yahweh/Jehovah in the DeMille classic The Ten Commandments? Of course not, but the scenes and presence were still pretty awe-inspiring.
You know, if Galactus is so damn powerful, why did he ever need to speak to the Fantastic Four (in the original trilogy) anyway? Let the Surfer earn his keep and be the intermediary.
An unseen Galactus is really the only way to go - makes sense from an artistic perspective and a financial one.