Doesn't work.Thanks! This works in 1.1.4 too! Here is a guide. Its basically yours, I just tweaked it abit.
1. You must have a 1.1.3 or 1.1.4J ailbroken iPhone with SSH and BSD Subsystems.
2. Navigate to /System/Library/Carrier Bundles/Unknown.Bundle
3. Copy the carrier.plist to your desktop and create a copy of it
4. Convert the plist file here so you can edit it.
5. Search for: <key>AllowPrefixMatching</key> <false/> (there should be more
then one... Just take the first one.)
6. Replace <false/> with <true/>
7. Right under AllowPrefixMatching, there should be a <key>CarrierName</key>. This carrier, for me, is Vodafone. It might be different for you. You can actually edit any carrier name, be it AT&T, O2 or T-Mobile. Just make sure you set AllowPrefixMatching to True.
8. Replace Vodafone (or whichever carrier you picked) with your carrier. Note that this is case-sensitive.
9. Replace the Default and FSO.png of the carrier you change. So, if you changed Vodafone, you got to replace the Default and FSO Vodafone png.
10. For the logo that appears in the springboard, it is the FSO.png. For the logo that appears in applications, its the default.png. Once you replace the images, restart your iPhone.
This is my jailbroken and unlocked 1.1.4.
Note that if you go to another country and have roaming enabled, it will still show your custom image so you might want to take note of this setback.
Guess I'm doing something wrong..