It depends on the device and the requirements of the devices' carriers. Most carriers specify that devices has bootloaders that will only allow flashing system

It depends on the device and the requirements of the devices' carriers. Most carriers specify that devices has bootloaders that will only allow flashing system images signed by the carrier; these devices require some manner of exploit to gain root access and overwrite the system's bootloader. There are also devices capable of running Android that have no restrictions on the bootloader at all, such as the Neo Freerunner; almost as "open" is the Nexus One, which has a bootloader with a user-accessible method to "unlock" the phone and flash a new system image at the cost of voiding your warranty.