I have been struggling with this one quite some time untill I figured it out. Since 1.4.2, the attributes to show in the admin's customer's form have to be in table customer_form_attribute.
You can add them with an upgrade in your module's setup, with this code:
$eavConfig = Mage::getSingleton('eav/config');
$attribute = $eavConfig->getAttribute('customer', 'your_attributes_code');
$attribute->setData('used_in_forms', array('adminhtml_customer'));
$attribute->save();
Hope that helps.
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