After a lot of trial and error and research, I've managed to update CCK fields programmtically. However, I cannot update from code the value of a radio button whose options are defined as pure text (value == label):
option1
option2
instead of values and labels:
1|option1
2|option2

For the 2nd case, I can update the radio as follows:

  $mynode->field_radio1[0]['value'] = 1; //works all right
  node_save($mynode);

But for the 1st case, the following is not working:

  $mynode->field_radio1[0]['value'] = 'Electrónico'; //loads null in content_type_mytype table
  node_save($mynode);

¿Would anyone know of a solution for this?

In case it may be of any use to someone, this is the code I use for updating dates and user-references:
Update date CCK field:

  $mynode->field_mydate[0]['value'] = date("Y-m-d\TH:i:s",strtotime($mail['headers']->date)); 
  node_save($mynode);

Update user reference CCK field:

  $mynode->field_myuser[0]['uid'] = $user->uid; 
  node_save($mynode);

Thanks in advance and regards!