As far as I can tell this is not possible with the current unique index, but it would be great to be able to unique-index just the keys of an array. This would really help in de-normalization as the following example could act as both a user table and products table if the key of the products array were unique indexed so that keys are unique between all documents in the table.

array(
'_id' => 1,
'name' => 'Rita',
'userdata' => array(...),
'products' => array(
1 => array('name' => 'cube', 'style' => 'polka dots'),
2 => array('name' => 'tiles', 'style' => 'checkerboard')
)
)

array(
'_id' => 2,
'name' => 'Joe',
'userdata' => array(...),
'products' => array(
3 => array('name' => 'circle', 'style' => 'round'),
4 => array('name' => 'triangle', 'style' => '3-sided')
)
)

With the current Mongo unique index I could still add the following even though the key already exists within another document:

array(
'_id' => 3,
'products' => array(
3 => array('name' => 'duplicate key', 'style' => 'key is not unique...')
)
)