A user field's text format defaults to "- Select -", how do I force it to an allowed option?
The text format is added in an after build function somewhere. This is off the top of my head so might need some tweaking, but the idea will be very similar:

function MYMODULE_form_some_form_alter($form, &amp;$form_state) {
  $form['field_bio']['#after_build'][] = '_MYMODULE_some_form_alter_textformat_options';
}

function _MYMODULE_some_form_alter_textformat_options(&amp;$element) {
  $element[LANGUAGE_NONE][0]['format']['#default_value'] = 'filtered_html';
}