the iphone can do this because if anyone can think back a long time ago if you used web apps (i did before i jailbroke my phone) that there was a web app called finger scanner and you could save your passwords and stuff online in this app and if you wanted to see them you had to scan your finger it was just a trial and to get the full version you had to buy it but you could scan your finger in the trial version and save it to your account and it really worked i tested it with all my friends and it would only work with me so this can be done i hope someone can look into this the link to this web app is
http://www.apple.com/webapps/enterta...ingerscan.html