The Outlook team has announced some changes in the way that add-ins will be handled upon closing Outlook 2010. This is to ensure that when a you close Outlook, Outlook will also exit as fast as possible.
Several changes to this behavior had already been made with Outlook 2007 Service Pack 2 but they will take it a step further in Outlook 2010. Especially when your add-in does anything important in the OnBeginShutdown and OnDisconnection methods of the IDTExtensibility2 interface or the ThisAddin_Shutdown method for VSTO add-ins, it is advised to read their guide and apply the necessary changes.
If you are a developer of add-ins for Outlook, also note that in order to support the 64-bit version of Office 2010, you’ll have to recompile your add-in specifically as a 64-bit add-in; 32-bit add-ins will not work on the 64-bit version of Outlook 2010.