A spell checker would be absolutely superb, but how it would be implemented for the end user is a difficult question.
When a user types a message, does it auto-check each word and alert of errors in real time, or does it display a dialog when Enter is pressed and displays all errors? Automation feature? As with the threat of killer automative robots, an automatic spell checker replacer could cause havoc (who's had fun with MSWord autocomplete?).
Don't get me wrong, it's a great idea, and put into mIRC wouldn't be that hard with electrik's suggestion of Aspell. I'm just worried about how it will be used in the end. Maybe we'll see it has a major feature with lots of options to satify the majority of users.