First you need to make your two images. They need to be 64 pixels wide by 20 pixels tall. I made mine in photoshop, but you can use any image editing program that works with png's and allows for transparent backgrounds. I will post a link to an example image. I was able to find some for t-mobile and at&t by searching google for "carrier png iphone". Make sure when you make your images the background is TRANSPARENT or it will not look right.
These images need to be named Silver_CARRIER_******.png and Black_CARRIER_******.png.
The ****** represents the numbers you see in the top left corner for your carrier.
example if you are on t-mobile in the usa then they would be named Silver_CARRIER_310260.png and Black_CARRIER_310260.png. I also created duplicate images named Silver_CARRIER-310260310260.png and Black_CARRIER_310260310260.png as sometimes my phone shows the carrier number twice when I'm connected to gprs. The duplicated images correct for that.
The silver image should have black or colored text and the black image should have silver or white text. This is so it matches up to the background color of the status bar correctly.
After you have created your images you need to SSH into you phone or use iFunBox. I personally use iFunBox so I will guide you using it.
Once iFunBox is loaded you need to click on Raw File System on the left pane. Then you need to navigate to System/Library/FrameWorks/UIKit.framework on the right hand pane.
Once you are in that folder you need to click Copy From PC on the top menu. Then navigate to your images you created and choose them. It will upload the images to the directory.
Next just respring or reboot and the numbers will be replaced by your images.
Here is a link to some example images I made for t-mobile:https://rapidshare.com/files/461144792/carrier.rar
Here is a video with the tutorial: