I just ran into the same problem. DW CS4 on XP winPro. Drove me nuts trying to figure out why the Insert, Update and Delete pages would go to the wrong URL throwing an error. I compared files with older admins I set up and found that by deleting the line:
if (function_exists("rel2abs")) $WA_redirectURL = $WA_redirectURL?rel2abs($WA_redirectURL,dirname(__FILE__)):"";
it works fine. Is there any reason not to just leave it commented out since it works?