Mark Update
Reuters is reporting (Yahoo News - British lawmakers back ambitious ID scheme) that Great Britain is going ahead with a new national ID plan. According to the report:

Britain's plans to launch the world's most ambitious biometric identity cards cleared their first hurdle on Tuesday, despite the first revolt against Prime Minister Tony Blair since his re-election...The cards would use biometric technology in fingerprint, face and iris recognition.

I think it is important to be aware of these trends in identification technology while noting that a very good argument can be made (The Mark of the Beast: Tattoos & Branding) that the mark will not be implanted technology at all, but could take the form of a digital watermark or even something as primitive as a tattoo.