As an amateur historian, I found the Colombus bit a bit interesting, and probably more on-point than Graham might have even known. Columbus, his backers, and his detractors all accepted that the world was round. What they disagreed about was how big it was, and how far it would be to Asia by sailing West. Everybody, pretty much, by that point knew that the world was literally round (and flat only in stories). This was especially true in monastic and church circles which had known this for longer.
In other words they all agreed it was a great idea and an ambitious project that might succeed. They disagreed about what it would take to get there, and whether there might be obstacles in the way.
Seems like a very fitting metaphor for an ambitious startup.
Edit: For sources, you can start with "Heaven and Earth in the Middle Ages: The Physical World Before Columbus" by Rudolf Simek, which is a book uncommon in its level of insight. His description of Marco Polo's purported encounter with a unicorn had me laughing in both humor and amazement.
Simek's basic thesis was that Columbus's expedition was important historically because it blew away an important piece of medieval ethnographic thought--- once it became clear that the areas he had reached were not India, but were inhabited anyway, it doomed the Augustinian argument against the existence of inhabited continents beyond Africa, Asia, and Europe. This then paved the way for questioning the religious and classical basis for some aspects of the physical world, and lead in many ways to the Renaissance (though I think the failure of the Crusades and the translation of Arabic writings into Latin had a strong hand there too). The importance of Columbus's voyage about changing the way we think about our place on the world was still important. Another good point about ambitious startups?