Seeing all these suggestions on a br2nl function, I can also see that neither would work with a sloppy written html line break.. Users can't be trusted to write good code, we know that, and mixing case isn't too uncommon.
I think this little snippet would do most tricks, both XHTML style and HTML, even mixed case like <Br> <bR /> and even <br > or <br />.
<?php
function br2nl($text)
{
 return preg_replace('/<br\\s*?\/??>/i', '', $text);
}
?>