I've blogged before about the need for shimming non-VSTO shared managed add-ins.  Misha Shneerson and Andrew Whitechapel created a COM Shim Wizard that worked with Visual Studio 2003.  They've now updated the wizard and it is available on MSDN. 

Article URL: http://msdn.microsoft.com/office/default.aspx?pull=/library/en-us/odc_vsto2005_ta/html/Office2003ShimWizard2.asp

Download: http://www.microsoft.com/downloads/details.aspx?FamilyId=300A6CDB-84DD-4C62-B3BD-E79194D83494&displaylang=en

You should use this shim wizard if you are building shared IDTExtensibility2 based add-ins for Office.  A shim gives you appdomain isolation and some other goodness. 

If you are developing add-ins for Outlook 2003, VSTO 2005 provides a better add-in project that you should use instead of Shared Add-ins.

If you are developing add-ins for Office 2007, the VSTO v3 CTP has add-in support for all the Office applications and so you won't need this COM Shim Wizard:  http://msdn.microsoft.com/vstudio/future

Want to work on the VSTO team?