A proper Object Model diagram is possibly the single most useful piece of doc for Excel programmers, not least because in a number of cases collections have untyped items, so it's not possible to work out from the library what the intended hierarchy is.
Sorry about the very long pause on this topic -- emergency events intervened.
Anyhow, I unmarked your response as The Answer because although it was helpful (thanks) it is only the very smallest part of the object model, not the full picture like what was available for 2003, and what would be available for 2007 if the formatting didn't
make it useless.
So, still hoping to either discover the real object model diagram, or prompt someone to produce it!