What Percentage of iPhone 4s Were Sold to Existing iPhone Users?

Despite the confusion surrounding iPhone 4 preorders, the lack of a white option, and craziness at Apple retail stores across the country, a ton of people managed to purchase an iPhone 4 this week. And since AT&T offered early upgrades to anyone with a contract expiring in 2010, I'm sure they expected many people to take advantage of the offer and upgrade from the previous 3G and 3GS models.

Can you guess what percentage of new phone sales were upgrades by current iPhone users?