You may have noticed that we've added some improvements to the Update Rollup installation process starting with Update Rollup 1 for Exchange 2007 Service Pack 2. This blog post walks you through the features we've added and what's happening behind the scenes. After reading this post you will have a better understanding about what may go wrong and how to tackle potential problems that may arise during the installation. We will have a separate post on how to analyze the errors for Update Rollup installations.
Read more at source: http://msexchangeteam.com/archive/2010/06/18/455213.aspx