Just FYI, you are not going to be able to use the camera for retina scans and use the mic for voice comparisons. They are way to shitty for that kind of thing (maybe the mic, but its going to be easy to fool). You will have to wait for the next tablet which will have a nice camera for that to happen.

And most people do not put data in the flash memory. It is too small and gets filled with apps. What you need is an encryption on the SD cards. It should (aka I am just talking here) be easy to port one of the various encrypted file systems to maemo, and use them.