The norm all over the tech industry is to have a beta before the final launch. Of course Apple likes to be different. This is probably the only case that I have ever read or reported about where a Beta is being launched after the final version. However in the case of Apple App store, its better late than never.
Apple has insisted that developers submit apps to them for testing. They test each and every app prior to letting it appear on the app store. As expected, its not easy. The Apple staff has been overwhelmed by the number of submissions leaving all the developers twiddling their thumbs.
As Craig Hockenberry of Twitterific explains, The big problem here is that the only way to install software on an iPhone or iPod touch is with the App Store. There are also no provisions for beta testing The only way to test a fix is to release the changes to tens of thousands of users. Its the developer equivalent of playing Russian roulette.
Even those developers whose apps havent been approved for listing on the actual store (thanks to the immense waiting list) will be able to put up their goods on the beta store.