Well, single on-off checkbox is a one useful feature in Drupal FAPI, it is also supported in Fields too. It appeared to be that the way to set the labels for those checkboxes is not well documented anywhere and is one of the most frequent asked questions.
So many think this is a core issue in Drupal but it is not.
For single on/off checkboxes, Drupal will use the on label specified in the allowed values field as follow.
0 1|This is your label