[Trent Adams]

<?php bb_stylesheet_uri(); ?> does work for me!?

Anyways, you can also do something like following I did in bbpress-mobile plugin:

<?php $stylesheet = (bb_get_option('uri') . 'my-templates/mobile.css'); ?>

<link rel="stylesheet" href="<?php echo $stylesheet; ?>" type="text/css" />

Trent