PS, I am sure there is a better way to do this and am hoping the horde 
team can do it better than my hack of a solution.

I can tell you what I did as its not that difficult.  In 
mime_drivers.php I added an if statment and changed the boolean value 
to be a variable name:


if ( $GLOBALS['prefs']->getValue('html_inline_viewing') === '0' ) {
         $viewHtmlInline = false;
} else {
         $viewHtmlInline = true;
}


/**
  * HTML driver settings
  */
$mime_drivers['imp']['html'] = array(
     'inline' => $viewHtmlInline,


and created a new preference in prefs.php:

$_prefs['html_inline_viewing'] = array (
        'value' => 1,
        'locked' => false,
        'shared' => false,
        'type' => 'checkbox',
        'desc' => _("View messages as HTML when both HTML and plain text are 
available (this change takes effect after log out)?"),
        'help' => 'prefs-notavailable'
);

You said you were modifying files for this - can you submit a patch?

Makes sense.

I am currently modifying the prefs.php and mime_drivers.php files to 
add the ability to give users the choice of viewing messages in HTML 
or text by default as a preference option.

Has the Horde team looked at adding this as a user settable option if 
inline HTML and text are being permitted (and a message has both 
formats being sent in message)?

Can this be added as a request for future releases?