Version 5.0.13 of Microsoft Dynamics CRM 2011 SDK has been released and can be downloaded here. The MSDN online version can be found here.
The version describes the December 2012 Service Release (‘Polaris’) features such as Extended Browser Support, Updated User Experience, Bulk Data Load and more.
If you want the really really short version, take a look at the SDK article What's New for Developers for Microsoft Dynamics CRM 2011 and Microsoft Dynamics CRM Online.
Happy reading!