The SQL Server 2008 Express package with Service Pack 1 is now available. The simplest location to install from is here, select “Download and Install” now. (Currently the other links on this package point to the original media. This will be revised soon). This tool will determine if the pre-requisites are installed and if not will install them for you. Next, the tool will install the edition that you have selected. This tool allows you to install SQL Server 2008 Express Advanced and SQL Server 2008 Express with tools, but only the Express package includes Service Pack 1.
For information on how to apply the SQL Server 2008 Service Pack to the Express Advanced and Express with Tool package see here.
Sr. Program Manager on SQL Server Development Team