Update Office Lately? Better check your VSTO...
OK... this may be old news to some, but I just had to figure out why the Office core dll was not being referenced correctly from the GAC when I created a new VSTO project. Seems that some recent updates to core Office 2003 have tripped up interop assemblies used by VSTO.
If you stumble across this too, the answer has been posted here by Paul Cornell on the official Visual Studio Tools for Office blog.
Thanks Paul for the update on the update :-/