If I understand this right the line you refer to seems to be the line 4618 in jquery.1.4.2.js in the style function. Which can only mean that the Autocompleter plugin tries to set a style value that IE8 doesn't understand or doesn't allow to be accessed/changed in this way.
style[ name ] = value; //style == elem.style from the passed in element
I have the exact same error on the exact same line, but for a completely different deal. That is, I'm not doing anything having to do with auto-complete; rather, mine occurs because I'm trying this in jQuery...
$(this).css('background', 'rgba(64,255,64,.4)');
Which jQuery tries to do...
style [ 'background' ] = 'rgba(64,255,64,.4)';
And it fails, of course, because rgba is not a supported CSS value for Internet Explorer. So you're not alone on this one, but in my case I was just doing it wrong. The appropriate jQuery syntax is this...
$(this).css({backgroundColor: '#40ff40', opacity: .4});
Here's my source...