Modding will probably be done in the same way we develop the game now. It seems very unlikely that we would tie ourselves to steam in any way. Steam would just be a another distribution channel.
Just use the "Reply to this topic" form on the bottom of the page, as it can get pretty annoying when you just want to read a topic but you have to scroll over the stuff you already read again and again.
Steam is geared more towards end users than people who are ready to test in-development software, so we want 0 A.D. to be in a better state before we try to get it on there. We will definitely look into it once we reach Beta though