A community technology preview of the first service pack for SQL Server 2005 is now available.  The CTP process, which was used for SQL Server 2005, proved to be such a good source of customer feedback, that the SQL Server team is continuing to use it with service packs. 

You can download the CTP from here:

http://www.microsoft.com/sql/ctp_sp1.mspx

The biggest change in SP1 is support for database mirroring. 

There are also a number of enhancements to SQL Server 2005 Express Edition including SQL Server Management Studio Express (SSMSE), support for full-text catalogs, and support for viewing reports via report server.   These changes are included in SQL Server Express with Advanced Services.  You can also download SSMSE seperately.

-David