Your identity is your most valuable possession, in every sense of the word "valuable". At the crudest level, anyone who steals it (i.e. can successfully convince financial and/or governmental institutions that they are you) can empty your bank accounts, put you into unrecoverable debt, commit crimes for which you may be held responsible and trash your credit rating, not to mention your reputation.
So it is not surprising that we invest a lot of effort in protecting and securing our identities. But can we ever succeed? Let's take a long view:
Any sort of card or other physical device can be stolen. Most such ID is now backed up by passwords, PIN codes or personal questions, all of which can be guessed or extracted by sufficiently ingenious technology. I suggest that each new security technology will be followed quickly by a successful hacking technology; I know of no exceptions so far. Readable tattoos or implanted RFIDs don't help, although they are a little more difficult to physically steal.
The next level is obviously biometric data: facial features, fingerprint and retinal image scanners are already in use at the "high security" end of the spectrum. Even supposing unhackable software is processing these data, there will soon be ways of simulating the real thing -- if there aren't already. It is not impossible to imagine DNA scanners fast and accurate enough to use for positive ID, but -- as always -- the ability to fake DNA can't be far behind the ability to recognize it.
There is also the problem of access to records of whose fingerprints, retinal patters and DNA are whose. We are understandably uncomfortable with entrusting governments and corporations (assuming charitably that there is a difference) with this sort of access to our identity and whereabouts, even if, as they always say, "we have nothing to hide." Moreover, if the archives exist, they can be accessed and even changed by sufficiently adept hackers. What would you do if DNA scanners suddenly started recognizing you as someone else?
Looking a little further down the technological road, suppose it eventually becomes possible to make a full scan of your brain, neuron by neuron, and that this becomes your ultimate ID? Will any entity that thinks like you, has your memories and believes it is you be considered by law to be you? And if not, why not?
I believe this is an entirely new class of legal, ethical and philosophical conundrum; but it is already in play. Best we think it through carefully and (if possible) rationally now, while there is still time to plan.