you have to download customize ipod fix from installer
OR
you can change it manually by SSH-ing into your iPod Touch and navigating to System/Library/CoreServices/Springboard.app
There there are two files called Default_CARRIER_IPOD.png and FSO_CARRIER_IPOD.png which are the carrier icons for when applications are opens and when you are on the springboard, respectively
Here are two apple images I made for mine to get you started