I had an old app icon in xcode that I had just attached to the program to see how it looked in the simulator. I was having the problem with the cocos2d app icon wouldn't go away. I made changes, deleted the images, and replaced them in Xcode. In my case I had kill the app before running it (thank you), delete the app from my device/simulator, clean the project, and clean the folder.
I'm guessing this is a problem only in development, it is likely that the cached icon is refreshed when an app is installed or updated from the App Store.
The answer turned out to be that my new icons had slightly different names than the old.
When I update an older version of my app pre-installed in the simulator, it updates the app just fine but the app icon is not updated. It's just a problem if the icon doesn't get updated on the current users' versions.
Even with cleaning and building the app ahead of time (sometimes a couple of times) this seems to be the case. EDIT: Still the same problem, but when I quit the simulator and start it again (NOT deleting the app), the icon is updated. Not an end-of-the-world kind of problem, but I would like to keep one standard icon for my app if possible.
Have you tried "upgrading" between two versions of your app with different icons in the simulator for i OS 5?
I am thinking it is probably a bug like nmagerko said..and set it in the Document Types section of my project, but my documents' icons in Finder remain generic.I've noticed that if I change my document type's file extension, the icon displays.A lot of my testing is just to make sure that all will go as expected with updates for current users.I've had this problem for a while and cleaning the project didn't work either. It works at least in i OS 6, I'm not sure in previous versions. The moment you move it there it should display the new icon, and then you can move it out again. If you're pushing out an upgrade to users and want the icon to change instantly, the only thing I've found works is to actually use a new file name for your icon. I am still seeing the issue a year later on i OS 6.1.3 on i Pad.but let me know if you find otherwise I ran into a similar problem after renaming the app and adding the Icon files category as described in one of the other answers.