We've been working on putting together docs on 4 SP1 features.  The documentation team will be ultimately releasing it through MSDN online docs updates but until then, it will be released through blogs.

• Work Item Custom Controls - Includes docs on the APIs and a sample.  Some additional docs are here.
• Extranet configuration - http://blogs.msdn.com/vstsue/pages/Walkthrough_3A00_-Setting-up-Team-Foundation-Server-with-Secure-Sockets-Layer-_2800_SSL_2900_-and-an-ISAPI-Filter.aspx and http://blogs.msdn.com/vstsue/pages/Walkthrough_3A00_-Setting-up-Team-Foundation-Server-to-Require-HTTPS-and-Secure-Sockets-Layer-_2800_SSL_2900_.aspx.  I have to admit that I haven't quite figured out yet why there are 2 of these but I wanted to get the info out as quickly as I could.
• Separating SQL/AS from the TFS server - Step by step instructions - but please note, these are not for the faint of heart. Unless you really need to do this, don't right now.  We'll make it easier in a future release - but at least now, it's possible.
• Detailed Merge history - Includes a basic explanation of the mechanics of the new API.

Enjoy!

Brian