The second generation iPad has sold amazingly well for Apple since its launch in early March. The tablet device is difficult to find in retail stores, and Apple's online storefront offers the iPad 2 with a 4-5 week ship time. Also riding this wave of success is the original iPad, whose supply will soon be depleted.
The first generation iPad is available in Apple's clearance website with a $100 price reduction. All models are available at this reduced price, except for the 16 GB Wi-Fi, which is apparently sold out. AT&T is also continuing to sell its supply of first generation iPads and only has the high-end 64 GB Wi-Fi + 3G available in its online storefront. This model is selling for an incredibly low $529.
If you want an original iPad, you'd better act quickly. Presumably, Apple has stopped manufacturing the first generation device and is focusing its resources on the second generation model. Apple is selling the remaining inventory from its last production run, and this stock will not be replaced when it is sold out. Of course, there is an ample supply on eBay or Craigslist, but the secondary market is fraught with dishonest sellers and many models will be used.