The Visual Studio team is designing the runtime components for Office 2010 so that your Visual Studio Tools for Office 2005 and Visual Studio 2008 .NET addins, document solutions and spreadsheet solutions will run on 64-bit Office 2010.  These runtime components will ship with Office 2010, so your end-users won’t even have to download a new runtime!  How easy is that?  There are a few rare exceptions that I’ll discuss in this blog entry.  The miracle of managed code allows you to write C# or Visual Basic .NET code that compiles to “Any CPU” using the Compile setting in your Visual Studio project.  Your code compiles to MSIL with Visual Studio, and then at runtime it gets JIT compiled to the correct chip set, either AMD, Intel, 32-bit or 64-bit.  The first exception to this wondrous technology is the oldest versions of .NET Framework 1.0 and 1.1 will not enable this 64-bit transformation. The other thing to lookout for is calls to process invoke (p/invoke) in your code

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Will your VSTO addin run on Office 2010 64-bit? Yes, probably. (Christin Boyd)