With Fake Operator tweak available in Cydia, you can rename your carrier to any name that you want.
The tweak works similar to the MakeitMine app that was released before, but with the difference that it don’t need a dedicated app 
to do so and is just a tweak that get’s added to the iPhone settings.
The app is supported on the latest Apple iPhone firmware iOS 4.2.1 too.
The tweak / app will not add any icon on the home screen. It can be configured by going into the iPhone settings app.
This app is not a complete replacement of MakeitMine but does the part with changing the operator / carrier name only.
Oh and did I forgot to mention, you need to have your iPhone or iPod touch jailbroken before this app can be installed.