This is a known problem with all versions of the ACS (and any other web site that uses return code 500 the way it is meant to be used).
You can actually turn it of in MSIE and get the error message. I forget the exact option, though...
Also, LONG error pages are correctly passed back to the user. Apparently, MSIE assumes a long page comes from IIS and is a "correctly formatted" error page, and doesn't need to be replaced.]
