Change is scary, and right now some people want to avoid 4e until it launches. I'm not one of those folks, but I understand it. So we'll make sure that for now 4e threads don't overwhelm the general forum.
Incidentally, "segregated away" is a far more negative connotation than I'd make -- this is to help keep all 4e threads in one place, and to make sure that other threads don't get drowned out in these first heady days. My guess is that we'll re-merge the new forum and the General forum at some point before launch.
We won't move 3e threads into their own forum; once the forums are merged at some point, they'll just share.