The iPhone SDK is so large, it's easy to miss things that other people think are obvious. Because of that, I'm going to post another tip that most people probably know already, but that some people might now.
If you create a PNG file of size 320x480 and call it "Default.png" (spelled exactly that way, making sure you have the first letter capitalized), you can then add that image to your Xcode project's Resources group and it will act sort of like a splash screen for our application - it will show that image until your application is fully loaded.
Although I'm not a big fan of splash screens, it's a good idea if you've got a slow-loading application to let the user know everything is proceeding okay. If you've got a slow-loading application, and you don't know why, it may be because you have a lot of resources in your project. The iPhone seems to do some kind of check on application launch, and the more individual files you have in your application bundle, the longer it takes. The actual size of the files doesn't seem to matter, but the number of files definitely does. If you're in this situation, consider reducing the number of files in your bundle somehow. A fairly easy way to do that is to store all those files as blobs inside a single SQLite database. You can use a read-only database from your bundle without copying it to the Documents folder, but you must make sure not to write to it, because if you change anything in your application bundle, code signing for the application will stop working and the user will not be able to launch it again. You can use a separate SQLite database in your application's Documents folder for user data.