The capacitive screen on the iPad is not sensitive enough to detect the fine gradients needed for fingerprint recognition. The design of capacitive fingerprint scanners is completely different.
The camera, as well, is not designed to recognize a finger print. Optical fingerprint scanners need special lighting and a surface to press the finger against to work.
Iris recognition is probably feasible, but the normal focal distance of the iPad's camera makes it awkward, if not impossible, to get a focused image that is also large enough to use. I tried. You could probably use some kind of lens attachment to fix this, but I don't imagine too many people would be willing to carry and use such a device.
Face recognition should be doable, and there is at least one iPhone app that attempts it; FaceVault. However the technology is still flakey even in business security and this app doesn't seem to be very reliable (going by the reviews)
Speaker recognition should be the easiest biometric to implement, but I can't find anything. Maybe I'm using the wrong search terms.
Thats all I can dig out of my memory or find in half an hour digging around. Sorry.
I found the
Wikipedia Biometrics article interesting, mostly it's See Also section that proved useful in identifying the various biometric methods in use and the sensors needed.