If you are building an app you probably are up to speed on this, but for anyone new to apps or coding them, you need to make sure your app is fully compatible with iPhone 3.0 software.  Apple has started to discriminate the apps that only work on the 2.2.1 platform and has stopped allowing them into the app store. Hopefully this means the release of 3.0 to the public with be very soon.  We would expect that the day or soon after WWDC that this will come to pass.