I recently upgraded a SQL Server 2005 Express installation to 2008 Express (with advanced).

My gotchas were:

1)Permissions on the new reporting services installation - much fiddling required to allow anonymous users to render reports without being prompted for username/password

2)Reporting services on port 8080 - I had to make changes to the web app to allow a port to be specified in report URL's, I also had to get port 8080 opened on the firewall, because the server in question was locked down to only allow traffic on port 80.