I just wanted to share how to install SQL Server 2008 Service Pack 1 from the command prompt.
This is a great guideline on How to: Install SQL Server 2008 from the Command Prompt. I was just missing the SP1 part.
So, you must download the SP1 installer from http://www.microsoft.com/downloads/details.aspx?FamilyID=66ab3dbb-bf3e-4f46-9559-ccc6a4f9dc19&displaylang=en
Then, unzip its contents.
Then, use the following command:
setup.exe /QS /ACTION=Patch /AllInstances /IndicateProgress >> log-sql2008-sp1.txt
The ">> log-sql2008-sp1.txt" will give you a detailed installation log, just in case...
You've been upgraded to SP1. Hope this helps someone!