Query Binding Not Replacing '?'

[eluser]hightower[/eluser]
I have the following code:

Code:

$sql = "SELECT articles.body, articles.user_id, users.username FROM articles, users WHERE ? = 1 AND articles.user_id = users.user_id ORDER BY ?";

$query = $this->db->query($sql, array('item1', 'item2'));
echo $sql;

The output from the echo however is:

Code:

SELECT articles.body, articles.user_id, users.username FROM articles, users WHERE ? = 1 AND articles.user_id = users.user_id ORDER BY ?

It's not replacing the question marks. I've checked the code, and it looks ok, and I've searched the forums/userguide but can't find anything that might help.

Anybody got any ideas?