First, there's the insertion process:

The tags, injected into the back of the arm with a hypodermic needle, consist of a toughened glass capsule holding a computer chip, a copper antenna and a "capacitor" that transmits data stored on the chip when prompted by an electromagnetic reader.

Then there's the import of the program:

Degrading offenders in this way will do nothing for their rehabilitation and nothing for our safety, as some will inevitably find a way round this new technology."

Harry Fletcher, assistant general secretary of the National Association of Probation Officers, said the proposal would not make his members' lives easier and would degrade their clients.

Fletcher continues,

"This is the sort of daft idea that comes up from the department every now and then, but tagging people in the same way we tag our pets cannot be the way ahead. Treating people like pieces of meat does not seem to represent an improvement in the system to me."

Civil liberties groups warn, "the rest of us could be next."

Radio frequency identification (RFID) technology is unproven. The technology is actually more invasive, and carries more information about the host. The devices have been dubbed "spychips" by critics who warn that they would transmit data about the movements of other people without their knowledge.

....One company plans deeper implants that could vibrate, electroshock the implantee, broadcast a message, or serve as a microphone to transmit conversations. "Some folks might foolishly discount all of these downsides and futuristic nightmares since the tagging is proposed for criminals like rapists and murderers," [consumer privacy expert Liz] McIntyre said. "The rest of us could be next."