One thing I noticed right away when editing my SharePoint pages with IE8 was an annoying popup message that showed every time you navigate away from editing a page. “Are you sure you want to navigate away from this page?”: Want to know an easy fix for turning this message off? Check out this post: http://www.chakkaradeep.com/post/2009/07/11/IE8-to-SharePoint-e28098Are-you-sure-you-want-to-navigate-away-from-this-pagee28099.aspx
Some of the out of the box SharePoint page layouts use a <div id="footer"> in the middle of the page. If your custom branded master pages, css, or theme has a style called out for #footer that sets a crazy width or height for your actual site footer it will ALSO apply to this div that is in the middle of the page inside of a table cell. This will end up stretching your entire page and is really difficult to track down. You have been warned.