Managing email opt-in on subsequent form visits

Marketo "WILL" pre-fill the opt-in field value always upon form load. With that said I like the idea of using progressive profiling to hide this field after it is filled in by the visitor.

If that's not the case, I think the best option is to use progressive profiling and place this question first in the progressive profiling list of fields, with a "Number of Blank Fields" is 1. I believe that should work. Please let me know if you don't think it will.

Actually, this won't work.  Non-nullable boolean fields aren't a fit for ProgPro.

But there's no need to use ProgPro. A little Forms 2.0 JS will do it:

<script>
MktoForms2.whenReady(function(form){ 
  var formEl = form.getFormElem()[0], 
      formRowEls = formEl.querySelectorAll('.mktoFormRow'); 
      Array.prototype.forEach.call(formRowEls,function(rowEl){ 
        if (rowEl.querySelector('#OptedIn[value="yes"]')) 
          rowEl.style.display = 'none'; 
      }); 
});  
</script>

Here's how we do it.  When someone clicks the opt-in checkbox, we have a master smart campaign that processes all opt-in requests, as well as our double-opt-in process for Germany (where an email is then sent out to get a user to confirm their opt-in preference).  Within that campaign, we capture the date of the opt-in in a custom date field. 

Then for all subsequent visits, we use a visibility rule for the opt-in field within the form.  We include the opt-in date field as a hidden field (form pre-fill turned on).  If the date field does not contain a value, then we show the opt-in field.  The only issue is if you embed this form on a non-Marketo LP (since form pre-fill does not work).  And unfortunately, you can't use block field updates here since you're dealing with a boolean field (block field updates does not work for boolean fields).  Block field updates is preventing a checkbox from being checked (has never contained a value)

Like Dan, we also use a conditional visibility "progressive profiling" hack. I don't like how restrictive progressive profiling can be with ordering of fields, so we rely on if a hidden field has a value to show or not show an opt-in field.