Make wp_list_pages print slugs instead of titles

I would recommend using get_pages() rather than wp_list_pages() for this purpose.

The get_pages() function will return an array of objects of Pages, and you can use the array data to build your own HTML list. e.g.:

<?php
// Wrap in a function, for later output;
function wpse47989_list_pages() {
    // Get an array of pages using default arguments
    // See Codex for list of arguments
    $wpse47989_pages = get_pages();

    // Open the unordered list
    $page_list="<ul>";

    // Loop through the results, and output an HTML list
    foreach ( $wpse47989_pages as $wpse47989_page ) {
        // Open the list item
        $page_list .= '<li>';
        // Open the link
        $page_list .= '<a href="' . $wpse47989->permalink . '">';
        // Link anchor text (post slug)
        $page_list .= $wpse47989_page->post_name;
        // Close the link
        $page_list .= '</a>';
        // Close the list item
        $page_list .= '</li>';
    }

    // Close the unordered list
    $page_list .= '</ul>';
}
?>

Then, of course, you just output the function wherever you want to display the list. (Note: this would be a good candidate for a custom Widget. You could even pass the function get_pages() arguments as Widget options.)

You may need to manipulate the foreach output for styling/etc.