Codewise, something similar to the smilies implementation.
All you need is two icons emptybox.gif and checkedbox.gif. Add two keywords like <emptybox> and <checkedbox> in the markup language. Need I say more? :-)
I have implemented it here, but I'm just too lazy to upload the gifs. I can mail the whole stuff (diff + icons) to whoever is interested. -- AlainMellan?.