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I've finished the first version of my (admittedly basic) spellchecker and have submitted it as a core hack
here.
You will need to use a version of PHP with pspell support included, and have apsell installed on the server with the required dictionary.
If you plan to use a dictionary other than "en-british" you will need to edit the html/class/spellchecker.php file.
There is also a PHP file that
should update the configuration database table to include the "use spellchecker" option in the main preferences, though this is untested
.