The SQL Server 2008 Service Pack 1 updates an existing installation of all editions of SQL Server 2008 including SQL Server Express. The SQL Server 2008 Express (SQL Server Express) Service Pack installs new instances of SQL Server Express including the service pack or upgrades instances of SQL Server Express to the service pack.

The SQL Server Express Service Pack can also be used to upgrade instances of Microsoft SQL Server Express 2005 or SQL Server 2000 Desktop Engine (also known as MSDE 2000) to SQL Server Express. Service packs are cumulative and include previously released service packs and hot fixes.

What's New in Service Pack 1

The following features are new in Service Pack 1.

* Slipstream You are now able to integrate the base installation with service packs (or Hotfixes) and install in a single step.
* Service Pack Uninstall You are now able to uninstall only the Service Pack (without removing the whole instance)
* Report Builder 2.0 Click Once capability

Source: TechSurface