SharePoint 2010 now provides the ability to create custom error pages. There are some very good reasons for doing this, especially if you have an externally published site. Todd Carter has written an excellent post that describes some of these reasons in detail, along with sample code to register your own custom error page. I highly recommend reading it and understanding when and how to implement it. You can find Todd's post on this topic at http://todd-carter.com/post/2010/04/07/An-Expected-Error-Has-Occurred.aspx. Thanks Todd!