Usually when Apple launches a new product it is fairly soon also available from other retailers. Not so with the iPad. The iPad launched early April and 4 month later the only place to buy an iPad is Apple.
The iPad ships from the Apple Store in 3-5 business days. This indicates that there is a bit of tight supply situation. The big retailers Amazon (the ones listed are from 3rd party charging fantasy prices for international customers I suppose) and Walmart can only sell iPad accessories, but not the main product. Only Best Buy has the iPad in stores, but that is because they host in-store Apple Stores in select locations.
One advantage Apple has to not have other retailers carry the iPad is that they margin is all theirs. Maybe it will be even a more explicit strategy to offer new products only in Apple Stores for a prolonged period to maximize revenue. It is quite a nice luxury to have your own (highly trafficked online store and retail store chain).