iPhone Developer Network

here is the youtube video showing how

and some more instructions:
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** To change the logo, or change your ringtones, or anything in the
future, you have to "jailbreak" your iphone using the iphone hack found
here:

http://cre.ations.net/blog/post/custom-ringtones--sounds-on-your-iphone-using-windows

Then, to change your carrier logo, just Plug in your iPhone (after it's
be "jailbreaked"), close iTunes and make sure you have 2 65x20 .png
files in your c:/phonedmg directory on your computer.

The first logo is the one you see on the homescreen, case sensitive:
FSO_CARRIER_ATT.png
The second logo is the one you see everywhere else in your apps:
Default_CARRIER_ATT.png

Make sure they are transparent so they look nice in the toolbars of
Safari and stuff.

Then you just run "iphoneinterface.exe" and once it connects, do the
following commands:

** EVERYTHING IS CASE SENSITIVE **

mkdir /System/Library/CoreServices/SpringBoard.app/

cd /System/Library/CoreServices/SpringBoard.app/

then type: putfile
/System/Library/CoreServices/SpringBoard.app/Default_CARRIER_ATT.png

then: putfile
/System/Library/CoreServices/SpringBoard.app/FSO_CARRIER_ATT.png

Turn your phone off, unplug it from your computer, then turn it back on.