With a new tweak available in Cydia called Fake Operator, you can now rename your operator / carrier and put it as whatever you desire.
Similar to the MakeitMine application that was released after the first-ever jailbreak, but with this new tweak you can do so without a dedicated app, oh and it is fully support on the latest iOS 4.2.1 firmware.
The app will not add an icon to the home screen, but options can be configured from the iPhone Settings app.
MakeitMine in comparison had the extra option of changing what is displayed on the banner and is also available for free.
You will, of course, need to have a jailbroken iPhone or iPod touch to install it. Follow our step by step guide posted here to jailbreak your iPhone, iPad or iPod touch with Redsn0w on iOS 4.2.1, or with Limera1n on iOS 4.1 if you haven’t already.
Fake Operator is available now from Cydia under the BigBoss repository for free.