Prefill Registration Form Fields
Fixed it. I had to work with
['und'][0]['value']['#default_value']

So my new code is:

&lt;?php
    $fieldMap = createFieldMap();
    foreach ($form['registration_information']['register_entities'] as $name =&gt; $data) {
        foreach($form['registration_information'][$name] as $subname =&gt; $subdata) {
            foreach ($fieldMap as $salesforceKey =&gt; $drupalMachine) {
                $form['registration_information'][$name][$subname][$drupalMachine]['und'][0]['value']['#default_value'] = $contact[$salesforceKey];                                 
            }
        }
    }