Ryan McAvoy said:
To me, hearing/seeing the Dreamworks logo says...
"Congratulations you've just paid to see a movie that you were hoping would be really good but which will only mildly disappoint you"
;-)
Where as the Fox Fanfare always comes with the feint suggestion that you may (Despite your brain telling you that it won't) be about to watch a movie with Lightsabers and Pulse Rifles in it ;-)
Funnily enough, John Williams also wrote this theme.