[support] How to access a user id & nickname from PHP ?

Hi again, So I've build this CCK node with various fields, some of them are userreferences. I want to access some values from PHP (in a custom CCK computedfield), but it doesn't seem to work: - I can access e.g. the date from my field_date using $node->field_date[0]['value']; - If I try to access a user from field_proprio, I get nothing. I tried $node->field_proprio[0]['value'] and $node->field_proprio_uid[0]['value'] but none of them work. Could anyone tell me how I can access a user, or which bit of the copious Drupal doc I should look at ? Thanks, Xav