stated recently that they didn't show more of Galactus in the film because they wanted to save him for the stand alone Silver Surfer movie.
Tim Story said the following regarding the devourer of worlds:
"We decided early on that we would not bring him to life completely as you might think of him in the comics, being a man with the suit and the helmet and everything else," Story said during his commentary. One of the main reasons is because we always knew that there was going to be a Silver Surfer 1, or they were going to make a spin-off of the Silver Surfer. If I gave up everything in this movie, then that movie wouldn't have much to go on. Knowing that I wasn't going to do it that early, I wanted to keep it fairly open to interpretation by the director who would do that one. So the idea of the cloud and everything made pretty good sense to us."
Kind of nice of him to be so concerned for the next director, don't 'cha think?
Right now everything is up in the air due to the possible impending stirkes in Hollywood, but the last I heard, Alex Proyas was set to direct Silver Surfer. I'm a big fan of Proyas and would thoroughly look forward to his vision of the silver dude on the big screen.
Source: Comics Continuum via Comics2Film
Fantastic Four: Rise of the Silver Surfer will be available on Tuesday, October 2nd