I've posted these same instructions like 4 times now :P
If you don't have AT&T replace AT&T with your carrier.
1. Make the new images in Photoshop (although I used Pixelmator).
2. Export the images to PNGs.
3. Open the image in Preview, and make sure the default opening Program is not the application you used to create the image with. (Important)
4. The names have to be exact, even capitalization, they are Default_CARRIER_ATT.png and FSO_CARRIER_ATT.png
5. The default image is when the backround will be white, and the FSO is when the backround will be black.
6. Navigate to /System/Library/Carrier_Bundles/ATT_US/ and backup the original files.
7. Replace Default_CARRIER_ATT.png and FSO_CARRIER_ATT.png in ATT_US with the ones you made.
8. If everything was done right, when you restart your iPhone, you should have a custom logo.
Result:
