This is a big upset, no doubt, and may be in the top handful, but I gotta agree with all of you who are saying that this isn't likely the biggest of all time. I suspect Alan thinks it is primarily because the wound is so fresh that it's hard at this time to imagine any other wound stinging more, which I can fully understand.
Part of the reason I don't think it can go down as the biggest upset is because of when it happened. If an FBS team goes an entire season as a top team, and then gets knocked off by an FCS team, it means they pulled off a major upset. On the other hand, if a team loses it's first game this way, there's a decent chance it simply means the team was drastically overranked. Frankly, how major this upset was will in part be determined by how Michigan performs the rest of the year.