In just over five days since the release of the Apple iPad, it is estimated to have sold 556,382 iPads. This estimate comes from ad network Chitika, who is said to have come up with the estimate by counting how many unique iPad Safari browsers appear on its ad network and multiplying that by the percentage of the Internet that Chitika services. I'm not sure about the math, but considering Apple sold more than 300,000 units the first day the estimate by Chitika sounds pretty reasonable.
Additionally, Chitika provides a breakdown of iPads sold by state. California leads the rest of the US with 19.40% of the iPad sales, Texas and New York are a distant second and third with 8.14% and 8.12% respectively. Again, the numbers are just estimates.
Not bad for a device that fits some place between the iPod Touch and a MacBook. Though I'd love to see how well this would have sold had it been running OS X instead of the iPhone OS.