SQL 2008: Unattended install.

E:\Installers\SQL>Setup.exe /qs /ACTION=Install /FEATURES=SQL,Tools /INSTANCENAME=MSSQLSERVER /SQLSVCACCOUNT="NT AUTHORITY\SYSTEM"
/SQLSYSADMINACCOUNTS=".\Administrator" /AGTSVCACCOUNT="NT AUTHORITY\Network Service" /IACCEPTSQLSERVERLICENSETERMS

From this blog post

Also worth looking at is the official docs.

Also this blog on using a .INI file