Include HTML in theme_pager_next $text variable?
If the rest of the code is the same used in theme_pager_next(), then it is not possible to add HTML tags to $text.

This is because theme_pager_next(), and theme_pager_last() call theme_pager_link(). theme_pager_next()'s code is the following one.

if ($page_new[$element] == ($pager_total[$element] - 1)) {
  $output = theme('pager_last', array('text' =&gt; $text, 'element' =&gt; $element, 'parameters' =&gt; $parameters));
}
// The next page is not the last page.
else {
  $output = theme('pager_link', array('text' =&gt; $text, 'page_new' =&gt; $page_new, 'element' =&gt; $element, 'parameters' =&gt; $parameters));
}


theme_pager_link() then contains the following code.

$attributes['href'] = url($_GET['q'], array('query' =&gt; $query));
return '&lt;a' . drupal_attributes($attributes) . '&gt;' . check_plain($text) . '&lt;/a&gt;';


As it calls check_plain(), if $text contains HTML tags, they are shown as plain text. 

To achieve what you are trying to do, you should override theme_pager_link() with a theme function that doesn't call check_plain(). filter_xss() could be a better function to use in this case, but remember to pass in $allowed_tags the list of the tags you want to allow.

As side note, when you want to alter the content of the $variables array a theme function gets, in Drupal 7 you can use its preprocess functions. Every theme function has preprocess functions, and in your theme that is THEME_preprocess_pager_next(). (Replace THEME with the short name of your theme.)