I've seen this issue before a long time ago, it might be known by the aspell plugin devs - but feel free to check up on this with to them anyway ;-)
The most likely solution at http://ckeditor.com/forums/CKEditor-3.x/Spellchecker-Fails-IE9
To quote:
davebeyer
In our case, this wound up being the result of code copied from plugins/aspell/spellerpages/spellchecker.html. Switching references to "opener" (apparently undefined in IE9) to instead call openerWindow() (see below) fixed our problem.
guikubivan
In our case, in addition to modifying spellchecker.html, we had to insert a similar function in controls.html and again, replace references to "parent.opener" to openerWindow()
fonji
I had to do both modifications and change a little the openerWindow() code:
function openerWindow() {
if (opener && opener.speller)
return opener;
if (parent.opener && parent.opener.speller)
return parent.opener;
if (parent.parent && parent.parent.speller)
return parent.parent;
return undefined;
}