Have you ever had this error message on your iPhone while trying to download an application or a game: Unable to download application.
There is a pretty easy fix for this and here is how it works:
Simply double tap the icon of the application that gives you trouble and wait for a few seconds. It should then be available.
If it does not work, the problem may also be that the app didn’t get downloaded completely. So, remove it and try downloading it again. I found it much better to download app over wifi, rather than 3G.