This is well documented in Apple's developer documentation and can be overridden with a custom loading image:
> When the system launches an app, it temporarily displays a static launch image on the screen. Your app provides this image, with the image contents usually containing a prerendered version of your app’s default user interface.
Er, almost. Your app provides a "Default.png", which is shown at initial app launch. This often contains either a loading image or a prerendered screenshot of your app with a blank state. However, since iOS4 and "backgrounding" (which imho is a better example or using hacks to give the appearance of speed), Apple will also dynamically snap a screenshot of your app at the time you close it, so that re-opening it feels instantaneous and more like "switching" to an app.
> When the system launches an app, it temporarily displays a static launch image on the screen. Your app provides this image, with the image contents usually containing a prerendered version of your app’s default user interface.
- App Launch (Default) Images: http://bit.ly/LMLbu0
It's been this way for close to 5 years now.