Theres a new category in the plist file called Icons Files (iOS 5.0). Try adding those file names to your target info like so:

In this tab:

The names and size much also be exact. See this Apple Technical Q&A #1686:
57x57 - Icon.png
114x114 - [email protected]
72x72 - Icon-72.png
50x50 - Icon-Small-50.png
29x29 - Icon-Small.png
58x58 - [email protected]
