[trixster]

It looks like there's a new folder where the pngs are stored as well..

Here's what I did

SSH into iPhone

Make backups of existing files by.. (keep the quotes to handle the spaces)

cd "/System/Library/Carrier Bundles/ATT_US.bundle"
cp Default_CARRIER_ATT.png Default_CARRIER_ATT.png.orig
cp FSO_CARRIER_ATT.png FSO_CARRIER_ATT.png.orig

cd "/private/var/mobile/Library/Carrier Bundles/ATT_US.bundle"
cp Default_CARRIER_ATT.png Default_CARRIER_ATT.png.orig
cp FSO_CARRIER_ATT.png FSO_CARRIER_ATT.png.orig

Now upload your custom png files to:
"/System/Library/Carrier Bundles/ATT_US.bundle"
AND
"/private/var/mobile/Library/Carrier Bundles/ATT_US.bundle"

Remember, the files MUST have the same name as the existing .png files

STOP HERE, RESPRING

If you ever want to restore the original carrier logos..

cd "/System/Library/Carrier Bundles/ATT_US.bundle"
cp Default_CARRIER_ATT.png.orig Default_CARRIER_ATT.png
cp FSO_CARRIER_ATT.png.orig FSO_CARRIER_ATT.png

cd "/private/var/mobile/Library/Carrier Bundles/ATT_US.bundle"
cp Default_CARRIER_ATT.png.orig Default_CARRIER_ATT.png
cp FSO_CARRIER_ATT.png.orig FSO_CARRIER_ATT.png