Instead of hacking it into my board I recommend
http://www.iespell.com/ to my users. It's a free plug-in for Internet Explorer that's fully customizable by the user. That way it's not affected by board upgrades or hacks, doesn't draw site resources, is generally faster, and is not limited to just the forum but works for any text entry fileds in the browser... forums, submission forms, webmail, etc. Been using it for 6 months now and will use it over any built in spell check options now simply for the speed and ease. You can either right click or add a button to the IE tool bar.