Now that you can add apps to the iPhone it'll only be a matter of time before you want to delete apps from your iPhone. To delete an iPhone app find it on your home page and put your finger on it and leave your finger there. After a couple seconds all the app icons will start to jiggle. For apps that are installed from the app store, there will be a little 'x' in the upper left corner of the icon. Tap the 'x' and you'll get a confirmation screen verify that you really want to delete the application, confirm and it's gone.

Of course this works for the iPod Touch too.

If you still have this application listed in your applications in iTunes on the computer that you synch to, you'll need to remove the application from there too. If you don't, next time you synch up the application will get installed again.