Great add-on!!!
I thought I would point out three changes I made, that you might consider for the public release. Minor usability tweaks.
1) Since SPELL_CHECK is already defined in the addpost.php, newpost.php and newreply.php language files, but as 'Check spelling', I changed those to 'Check Spelling' (note case) and also added SPELL_CHECK to generic.php with the same---keeps it consistent across all pages.
2) Changed popup width to 500 pixels, height to 550. Bigger is better.
3) Added the spell check button to lines 66 and 126 in addpost_preview.tmpl.
