Before Windows Server 2008 even hits, people are blogging about the breaking changes for older apps.  some of the stuff is pretty fundemental so a 25 point blog on the subject is semi-worrying :¬(

Read these before upgrading your IIS version.  Depending on the complexity and topology of your application, there could be afair few changes required to get it working on the new version of iis

http://mvolo.com/blogs/serverside/archive/2007/12/08/IIS-7.0-Breaking-Changes-ASP.NET-2.0-applications-Integrated-mode.aspx