I went back to where I was saving created_by in a hidden field and used the Insert Record behavior to have the hidden field point to the form's email field that was supplied by the user so that seems to have taken care of that problem (it defaulted to using itself as the value to be inserted which of course there was no value).
Regarding the created_dt hidden field, you mention "you can just set the value directly in the insert server behavior." Where is the insert server behavior value you are mentioning? Is this where it shows the value with a lightning bolt in the DW Properties inspector if you click on the hidden field?
Also, I'd like the created_dt to have a value like "2009-09-02 22:47:05" to be inserted in the database. Do you have an idea of what that php code would look like to associate it with the value of the created_dt hidden field?
thanks for your help. hopefully others will stumble on this and find some value in it.