Autodesk Developer Network 2011
Autodesk
Revit 2012 SDK
§The SDK is provided with the product
§From Installer under “Install Tools and Utilities”
§Web and download version
§<extraction folder>\Utilities\SDK\RevitSDK.exe
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§SDK Installer in Revit 2012 RTM temporary setup files
§C:\Autodesk\Autodesk_Revit_Architecture_2012_English_Win_32-64bit\Utilities\SDK\RevitSDK.exe
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§Latest SDK update will be posted to Revit Developer Center
§http://www.autodesk.com/developrevit 
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The Revit SDK is basically purely for support and documentation purposes.

The SDK install is located under the 'Install Tools and Utilities' menu on the main page of the Revit installer.

Alternatively, you can also find the SDK installer in the Revit 2012 temporary set up files under:
C:\Autodesk\Autodesk_Revit_Architecture_2012_English_Win_32-64bit\Utilities\SDK\RevitSDK.exe

The latest SDK updates are always posted to the Revit Developer Center and should be downloaded from there to get the latest.