Is it much of a mission then to downgrade from:

VSTO 2008 to VS2005 SE to get an Outlook 2007 Addin to work on Outlook 2003?

I can think of one difference -- the ribbon. Apart from that is there a
definative list anywhere that lists all the differences?

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From
http://www.microsoft.com/downloads/d...isplaylang=en:

Overview

This download (VSTOR30.exe) installs the Visual Studio Tools for the Office
system 3.0 Runtime, which is required to run VSTO solutions for the 2007
Microsoft Office system built using Microsoft Visual Studio 2008.

When installing the VSTO runtime on a non-English version of the operating
system, you should also install the appropriate language pack to see the
text of the messages from the VSTO runtime in that language, rather than
English.

Note: VSTO solutions built for use in Microsoft Office 2003 require the
Visual Studio 2005 Tools for Office Second Edition Runtime, not this one.