A few weeks ago I started a beta program for my iPhone application using the official Ad Hoc distribution model, allowing my beta testers to install early versions of the application by simply dragging it into iTunes. It's all very convenient, but it bothered me that my application showed up under the Applications section of iTunes with a boring black generic icon.
Luckily there is a simple fix for this.
What you have to do is prepare a 512x512 pixel JPG image, rename it iTunesArtwork, without extension, and then copy it into your application bundle. You access the contents of your application bundle by right clicking the .app in Finder and selecting Show Package Contents, then just drop the image in there next to all the other stuff making up your application.