In many ways I think that thirdeg has hit the nail on the head. National ID cards will only catch the inexperienced criminal - the more experienced ones will actually exploit the system to their own ends. And terrorists will either ignore it, or use convincing fakes if they need to.

Just as a minor point to jared_c, as far as I know there is no back door to PGP (regarding the maths behind it - I do have a maths degree), which is why the UK & US governments want to get everyone to change to a a system where everyone has to register their keys with them.