I didn't know this existed: http://www.guidancelibrary.com/
Apparently, it's by our Patterns & Practices team and has all of the latest guidance documents with regards to security.
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There is a lot of information on the Microsoft site (or sites owned by MS). Unfortunately they are all so scattered and the search engine does not bring all these results.
Check out the Guidance Explorer: http://www.codeplex.com/guidanceExplorer. It aggregates a lot of the best practice security guidance patterns into a single location.