Superman / Clark Kent : Michael Trucco from Battlestar Galactica. He's 6"3 or something and has the looks, just needs to bulk up a bit. Seems to be a decent actor too but not well known enough yet to overshadow the film
Lois Lane : Evangeline Lilly. Yes, everyone else has picked her too but she has it all - she can act, she's sexy, and she's tough ... while also being able to pull off that innocent doe-eyed look that a good reporter needs to get out of situations.
Jor-el : Jonathan Frakes from Star Trek TNG. To me, Jor-El should be dignified, commanding but having a compassionate side to. Phsyically - he also needs to resemble Superman in height and looks slightly, although he's obviously older. Frakes could pull it off.
Brainiac : Gary Sinise. Can do cold and calculating with his eyes closed.
Jonathan Kent : Martin Sheen. I always thought he had the look of the comic book Kent.
Martha Kent : Glenn Close