Comic Book Resources has the full report on the panel, but here's an excerpt on Superman:
"Which did Morrison like more: writing Batman or Superman? Morrison said while he goes back and forth, he was 'buzzed' to be writing Superman again. 'The capitalist millionaire seems to have a little less to say to us now than the champion of the oppressed,' laughed Morrison.
"Explaining that his Superman in Action Comics would be a more mischievous, 'anti-authoritarian' character, Morrison reveled that Clark Kent would become almost as big a name in Metropolis as Superman. 'He's a social reformer, and Clark Kent does as much work as Superman basically uprooting corruption and exposing corruption, so the two of them are working in tandem,' said Morrison. Elaborating that in his version Ma and Pa Kent are dead, Morrison told the audience his Superman will be more of a mischievous character. 'Justice may not involve the law in Superman's eyes,' added Morrison."
Additional information revealed includes that his plan is for a 16-issue run on Action, that although the comic initially takes place five years in the past, by issue #7 it will catch up to the current timeline, from which point stories will jump back and forth in time; and on why people prefer Batman over Superman, he muses that Batman is cool, sexy and has a lot of money, while Superman has a boss, works in an office and is "shackled" in that he has to represent the flag and patriotism.