Since, with this system or any other, someone at some point needs to actually look at an ID, it seems to me that a purpose-built cert that indicates that this was done would be better. Then it would retain the same attributes (aside from being able to be used for anything beyond validating you're over a certain age), you could store it as a file and use it without any special hardware.
Another thought: what about people who use multiple machines? Since you couldn't share credentials across machines, wouldn't that require users to go through the enrollment process for each machine?
