I would just forgo the launch image for Instapaper then. Just get the UI up as quickly as possible. Also, supporting "multitasking" on iOS 4 helps with getting the UI up quickly when brought into the foreground.
You can skip the Default.png file. The experience without it can be terrible, though. Your app will just show a black, blank screen until it finishes booting.
It's a bad enough experience that it's not even worth considering.