This article has information on all the command line switches for SQL Server 2008 including Express. Details including required/optional switches, commands superseded from previous versions, repair/rebuild/uninstall, etc.

http://msdn.microsoft.com/en-us/library/ms144259.aspx

Here is the command I used:

SQLEXPR32_x86_ENU.exe /QS /ACTION=Install /FEATURES=SQL,SDK /INSTANCENAME=MSSQLSERVER /SQLSVCACCOUNT="NT AUTHORITY\Network Service" /SQLSYSADMINACCOUNTS="Users" /AGTSVCACCOUNT="NT AUTHORITY\Network Service"