A new version of the Custom Domains SDK is up at Microsoft.com downloads, version 1.2.  Here's the overview:

The Windows Live Custom Domains SDK is intended for customers and partners who want to programmatically accomplish many of the administration tasks that are available on the Windows Live Custom Domains Web site in addition to tasks that are not available on the Web site, such as importing and exporting user lists.

The SDK uses the Simple Object Access Protocol (SOAP) remote procedure call (RPC) 1.1 standard for communication between client applications and the Web servers for Windows Live Custom Domains.

The SDK consists of the following:

1. A command-line application and code samples (in Microsoft Visual Studio C#).
2. A Win32 user management application with source code (in Visual Studio C#).
3. Documented SOAP calls for each function

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Custom Domains, SDK Updated