android:label parameter is displayed just under your icon on the home screen, but I can't see where the android:description label is displayed on the phone (Home screen, Applications Settings menu,... ?).
I didn't find the answer in the Android official documentation.
Any idea?
This is an excellent question, and I likewise can find no answer.
The value for android:description does not seem to appear in app selection screens (for generic intents). There is no option in the app tray to view more details about the application. It does not show in the App Manager. The Android Market description is set through the Android Market website when someone publishes an app and does not seem to come from the app itself.
I thought that, like android:title, it may show only in certain themes. However, none of the themes that I see in http://developer.android.com/resources/samples/ApiDemos/res/values/styles.html or http://developer.android.com/reference/android/R.style.html seem like they would include a description.
The only other possibility I can think of is that it was added "just in case" they wanted to use it in some app or version of Android later on... and that they never got around to it, making it useless for the time being. Unfortunately, we may never know, unless someone from the Android team itself talks.
Summary: as best as I can find, android:description does not show ANYWHERE.