You don't have to use winterboard. You could always go into the folder where the carrier logos live and swap them out directly. Here's one such path: /System/Library/Carrier Bundles/Unknown.bundle That's how I got SimpleMobile graphics on my iphone 4.
hackthatphone is right. Carrier logos are just .png graphics stored in the carrier bundle, and can be swapped out directly. Most carrier logo packages I know of work this way.