You can grab it on the Knowledge Center. Make sure you use the new EPiServer CMS Manager.

One of the new things in the SP1 release is the Content Channel Service, which can be used to feed and synchronize your EPiServer content from external systems. This feature is used by the Sharepoint Connector, but is a general implementation, and can be used by other services too. Read more in the tech note.

If you have written your own role or membership providers, be sure to read the release notes, as the ProviderCapabilities class has changed.

The way the profile provider in ASP.NET 2.0 is used by EPiServer has also changed. This is a breaking change if you have code that relies on the HttpContext.Current.Profile being an EPiServerProfile class. From SP1, it is not. Instead, all the profile properties that EPiServer uses has been added to the profile configuration, but the profile provider does not have to inherit from EPiServerProfile. This give you added flexibility, and the profile can now be stored anywhere.

Also, the Event Management System has been changed (and simplified). If you want to configure cache invalidation, or use the event system to communicate your own data between sites, have a look at the new tech note. The event system uses Windows Communication Foundation, and there is a diagnostics tool available if you need to troubleshoot this (see the tech note).