trackback_rdf produces no output

Hi,

I’m using trackback_rdf() inside my theme-templates (index.php and single.php), within the loop:

<!–
<?php trackback_rdf(); ?>
–>

It seems, that the function trackback_rdf() does not produce any output. When I view my blog’s HTML-source with my browser (firefox), the line where the rdf should appear is empty:

<!–

–>

Trackbacks and pingbacks are enabled on my site. I’m using WP 2.2.

I tried other themes, trackback_rdf() does’nt work there, too.

Does anyone have an ide where the problem could be?

Thanks!
Stefan