We are working on adding support for Visual Studio 2012. More information about this will be made available in the coming months.
We did install the SDK without VS2010 Express.
Just add the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\VisualStudio\10.0\InstallDir (REG_SZ) with the value "C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE\" and start the setup for the SDK.
You will see a folder "C:\Program Files (x86)\ArcGIS\DeveloperKit10.1\Microsoft Visual Studio 10.0" (assuming a default ArcGIS 10.1 installation). There you will find the templates, snippets and xml schemas. Just copy these to the corresponding folders of the VS 2012 installation.
The snippets should be registrated in the SnippetsIndex.xml.
Most things seem to work fine with VS 2012, but we have troubles to build Addins:
[ATTACH=CONFIG]18317[/ATTACH]
Has anybody made the same experience? There seems to be a hard dependency on the very old version 2.0.0.0 of the Microsoft.VisualStudio.Shell.9.0.dll.
VS 2012 does not support this old version.
It would be very, very nice if ESRI would support VS 2012.
For now the support VS2012 is in ArcGIS Runtime SDK 10.1.1 for WPF ( http://resources.arcgis.com/en/help/system-requirements/10.1/0151/015100000092000000.htm ) and ArcGIS API for JavaScript version 3.(1..3) (http://help.arcgis.com/en/webapi/javascript/arcgis/jsapi/#api_codeassist)