Can I change the carrier name in the iPhone Simulator 5.0?
For my part, I was able to change the carrier name for the iPhone 5.1 simulator by editing this file:

/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.1.sdk/System/Library/CoreServices/SpringBoard.app/French.lproj/SpringBoard.strings


As the path suggests, I'm French. Switch to whichever localization you choose.

Inside this strings file (editable with XCode if it's a binary plist), you'll find a SIMULATOR_CARRIER_STRING key. Change its value, fire up your simulator, and there you go.