Portugal already has these. Our ID card is a smartcard, and contains a personal X.509 certificate, issued by a national certificate authority (and a bunch more stuff, like my address or photo). You can use a card reader and standard software to sign legally valid documents. You can login into government websites with it.

I'd wager about 80% of ID cards already use the new model.

It looks like this:http://4.bp.blogspot.com/_4kQttk9aLQI/TT7-nale3lI/AAAAAAAAAC...

Its site is: http://www.cartaodecidadao.pt