On the quick comment box:
[post comment] [more options] [checkbox meaning: don't post comment when you click post comment (!!!) but preview and check spelling instead]
On the preview page:
[post comment] [preview] [checkbox meaning: also check spelling when you click preview]
On the New Entry page:
[Update Journal] [Spell check which also previews!] [Preview]
You know the jokes with the button you have to click but that keeps jumping to a different location when you approach it? Well, this isn't all that different, except this is not meant as a joke!