I think integrating a red-line spellchecker to the edit box would be a bit difficult (the line, not the spellchecker). And depending on a person's color choice for the edit box, it may not even be visible. Also keep in mind that mIRC's used by people all around the world and including dictionaries for multiple languages would be pointless, while just including an English one would be a bad idea as well.
I have to say that I think spellchecking would be best done as a script than as a built-in option in mIRC. With only a one-line edit box (most people don't use multi-line) and considering that chatting isn't meant to be perfect grammar and spelling, I really don't see any reason why it should be added.
Of course, if people could spell, there'd be no need for a spell checker.
