In the next release of SQL Server, called SQL Server 2008 R2 we are working on addressing issues that complicated the installation of SQL Server Express. As the timeframe to address these issues is tight for this release we won’t be able to address all issues. However, still take the time to file your issue at http://connect.microsoft.com/. If we can’t address in SQL Server 2008 R2, we will consider addressing in the next major release.
SQL Server Setup Team