Using spellchecker plugin in TinyMCE

I have installed PSpell and updated the config file to use it instead of GoogleSpell. Then in my functions file I have added spellchecker to the plugins part and the button. The button shows up but it doesn’t work. After looking at the source code I see that spellchecker is not included in the TinyMCE init.

function customize_tinymce($in) {
$in['remove_linebreaks'] = false;
$in['gecko_spellcheck'] = false;
$in['keep_styles'] = true;
$in['accessibility_focus'] = true;
$in['tabfocus_elements'] = 'major-publishing-actions';
$in['media_strict'] = false;
$in['paste_remove_styles'] = false;
$in['paste_remove_spans'] = false;
$in['paste_strip_class_attributes'] = 'none';
$in['paste_text_use_dialog'] = true;
$in['wpeditimage_disable_captions'] = true;
$in['plugins'] = 'inlinepopups,tabfocus,paste,media,fullscreen,wordpress,wpeditimage,wpgallery,wplink,wpdialogs,wpfullscreen,spellchecker';
$in['content_css'] = get_template_directory_uri() . "/editor-style.css";
$in['wpautop'] = true;
$in['apply_source_formatting'] = false;
$in['theme_advanced_buttons1'] = 'formatselect,|,bold,italic,underline,|,bullist,numlist,blockquote,|,link,unlink,|,pastetext,pasteword,removeformat,spellchecker,|,charmap,|,outdent,indent,|,undo,redo,|,wp_fullscreen';
$in['theme_advanced_buttons2'] = '';
$in['theme_advanced_buttons3'] = '';
$in['theme_advanced_buttons4'] = '';
return $in;
}

Viewing the source I see the plugins being loaded but not spellchecker.

ref : {plugins:"inlinepopups,tabfocus,paste,media,fullscreen,wordpress,wpeditimage,wpgallery,wplink,wpdialogs,wpfullscreen",theme:"advanced",language:"en"}

What am I missing?