I don't know if this has been addressed or not, but it would appear that you need to escape all characters in the to-be-spellchecked-text that will be interpreted by the browser's javascript parser. Namely a double quote anywhere in the text will screw up the javascript in the "Finish checking" code (at least on IE 5.1 on the Macintosh).
This is the javascript that gets generated (which fails on my browser) when using your test page over here (
http://spellchecker.killermovies.com/demo/):
<!-- KMC Spell Checker v1.1 (1)//--><html><script language="JavaScript">
parent.window.opener.ftext.value = "receive that'll be "a day!";
parent.close();
</script></html>
The input string was this:
receive that'll be "a day!
Note that the double quote in the text prematurely terminates the parent.window.opener.ftext.value javascript line...