I should also add that this same line contains a typo.
The original file in the plugin reads
restoreTitle: '" . __('Click to close image, click und drag to move. Use ARROW keys for previous and next.', HI_TEXTDOMAIN) ."',
und should be and, of course.
I found the solution in the first post thanks to the Highslide forums (Highslide problem in IE after upgrading to WordPress 3.0). The person who made the original posting said he had notified the author of the error last July. It's not been fixed yet though.
Anyone with the IE problem though can fix it themselves by going to Appearance => Editor in their WP Dashboard and editing the highslide-integration.php file. Remove the LAST comma in the line, not the one between the brackets.