First of all, the sidekick CAN be unlocked.

Yet as Char777 stated the data functions will only work with TMobile & Suncom's networks.
Second, Alltel is a
CDMA carrier, the sidekick is a
GSM device. This means they are incompatible no matter what you do.
I also want to clear up something Char777 said. According to the new law passed recently, it is no longer illegal to unlock phones, however, the carriers in no way
have to unlock the devices for you. And the carrier that you are trying to use the unlocked device with doesn't
have to allow you to use that device on their network.