The Windows Media Center team posted the RTM version of the Windows Media Center Software Development Kit 6.0 for Windows 7 to the following location.
http://download.microsoft.com/download/6/C/6/6C6B665D-6240-4958-A028-A944B8BD6860/WindowsMediaCenterSDK6.msi
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Kudos goes to Niall Ginsbourg for breaking the news.