I am having the same issue as Lisa.  I have read every entry in this thread and have done the things suggested and none of them work, even changing the payment terms 4 times and even changing the payment terms to net 60.  An invoice dated today is showing past due with the payment terms of net 60 selected.  I have made sure the Net 30 terms are correct by going to the terms list, right clicking net 30 and making sure 30 is in the Net due in...  box.  Nothing seems to fix this issue.