Differences between last year and this year...
Last year: Japan got hit with a Tidal Wave, followed by Nuclear Meltdowns this had a global impact on the electronics industry including Apple. Businesses and entire industries were swept away, and multinational industries had to scramble to find replacements. As such, Sony's camera line suffered.
This year: Thailand got hit by massive flooding at the end of last year, this disrupted "Platter" drives and caused more use of SSD drives; therefore, their prices have dropped in relation to platter drives as SSD are selling more. Also Sony suffered again, as their cameras were also being made here. This has also caused the margin for profit to drop on laptops/desktops which use platter drives as well as pushed the prices too high for people to "justify" those purchases. This has overall helped Tablet sales. Apple has also had record funds available all year long and has been able to fund a much better launch and initial build. Additionally the market is full of iPad and iPad 2 devices - the New iPad is really nice, but the iPad and iPad 2 are also nice tablets too. At the end of the crunch last year a lot of scalpers saw its own "dot com bust" and many were stuck with inventory if they did not have the receipts to return them.
I somewhat doubt we are going to have sellouts, unless the scalpers have deep pockets and not a lot of sense this year, or if Apple holds back. I ordered two, one to use and one to "try" reselling. I figured if it did not sell I could either return it or keep it for my wife (because of the cost, we're going to see if the boost is worth the expense before she opens hers - sold my iPad 2 already, and she does not want to resell her iPad 2 but use it for the kids who have an iPad 1). Additionally, the economy has been down for many people. Usually scalpers do so because they do not have a lot of funds and see an opportunity to make more. If you are stuck with something this expensive on a credit card, you are going to see your profits eaten by the credit card holders. Again, I do not think it is going to pay to be a scalper on these this year. Supply is higher than demand right now. Most of us die hards preordered ours and an extra, everyone else will just wait and get it in the store or get the extras from friends/family. If Apple did not have the extra supply they would not have done the preorder sells. They did them because they knew how much inventory they had and did not want to get stuck with it all (retailers get shafted, and Apple profits off preorders and the large supply requested by retailers). Also as pointed out, Walmart stores might be sold out but Walmart online has plenty and will ship to stores (helps to be able to send stock to the areas that need it, Walmart did their release smart).