Some of this material comes from a fascinating article in New Scientist (Sept. 17-23, 2005) entitled "Privacy & Prejudice: Whose ID Is It, Anyway?" (However, any uproarious one-liners -- assuming you notice them -- are mine).
Biometrics (in one form or another) is often considered a cure for identity theft, How about full X-ray skeleton recognition? Everybody has one. terrorism, illegal immigration, lost laundry tickets and so forth. There are, however, significant practical problems with its deployment. For example, people with worn finger pads (which often characterize certain kinds of manual workers) can't be enrolled in fingerprint recognition systems (nor--obviously--can people with lost limbs) and the total for worn pads is estimated at about 2 percent of the population. Further, we leave fingerprints wherever we go, so getting copies of other people's biometric identifiers would be straightforward. (DNA scanners [still in the lab] suffer from a similar vulnerability.)
Separately, one in 75,000 people have no retina, and in any case there are proposals on the table that involve using tinted contact lenses to spoof retina scanners. Voice recognition is vulnerable to ambient noise (can you imagine using it to enter a subway system?) and might be spoofable with a surreptitiously made recording (cf Sneakers). Proposals that rely on multiple identifiers have been deemed (in some quarters) too complex and expensive for mass deployment. There's also (I suspect) a voting problem: you put your hands in the fingerprint scanner, smile for the face scanner, speak your pass phrase for the voice scanner and stick your eyeball up against the retina scanner. What if some of those scanners (for whatever reason) reject you while others accept you? What if some of your fingerprints pass and others fail? What's the poor security guard to do?
The significant difficulties that attend most biometric technologies make my proposal for full X-ray skeleton recognition (which no one seems to take seriously) look very attractive. Think about it! Everybody has one; you don't leave copies of it lying around; it works in noisy environments; you can't be separated from it (well, technically, I guess a really determined thief...never mind) and it can't be imitated. For added security, you could even have a personal little "pass-dance" (maybe something from A Chorus Line) that you'd perform to further confirm your identity. How about it, science?!