if your using tmobile, you need to put in the unkown bundle under /system/library/coreservices/springboard. also make sure you change the names to match exactly what your replacing..I had the same problem but realized that in the unkown bundle isnt labeled default_carrier_att. It was something else if i remember correctly...
I made a post about this in file mods, it's got two locations now since the carrier updates.
EG, this was my old location: /System/Library/Carrier Bundles/Optus_au.bundle/
This is my new location: /private/var/mobile/Library/Carrier Bundles/Optus_au.bundle/