How do I change the carrier logo on my iPhone?

You can change the carrier logo on an iPhone by downloading the Cydia tweak Zeppelin. It's available for free in the ModMyi repo or just search "Zeppelin". The tweak works with all jailbroken devices.

Once installed go to Settings --> Zeppelin --> Theme and select the icon that you would like to replace your carrier logo with. Zeppelin comes with a few pre-installed icons (e.g. Apple logo, Pac-Man) to get you started, but there of plenty of Zeppelin Addons available in Cydia.

iPhone 4 Jailbreak Instructions (iOS 5.0.1 Redsn0w)
iPhone 4S Jailbreak Instructions (iOS 5 / 5.01 Absinthe Untethered)