As documented in the ArcGIS 10.1 Service Pack 1 "http://support.esri.com/en/downloads/patches-servicepacks/view/productid/189/metaid/1913", it says to get Service Pack 1 for Server on the Customer Care Portal. I logged in but the only reference I saw for Service Packs was on the EDN downloads page. I clicked that link and it took me back to the above link.
So where is the download for the ArcGIS 10.1 Service Pack 1 hiding?
Just confirmed with ESRI tech support that the ArcGIS Server 10.1 Service Pack 1 was not available for EDN subscriptions until just yesterday. They service packs should be there now for EDN subscriptions.