Sorry, no this not possible. Even putting a different carriers OS on the device will not change the carrier logo on start up and shut down. This is something that is programmed on a separate chip inside the device.
Unfortunately, the BB security is mucho better than dumb phones, the startup image is buried in a file that even a good hack will never access. Best you'll probably get is finding the file that contains it and removing it, which in all likelyhood will brick your phone. Good news is, bbs can recover from a good brickin..and I've bricked a few in my day.
Posted from my CrackBerry at wapforums.crackberry.com
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