They use this company for the verification process of ID scanning.
http://www.jumio.com/netverify/
I wonder if this has been brought on due to a friction of fraud that they are facing as they've grown. They've actually done a great job of keeping fraudulent activity under-hush, but I'm sure it's been a major problem...
Also, one of the main issues with a marketplace that fulfils the service/product at a future date is the uncertainty that the home owners actually exist. They've done a good job of limiting their exposure to risk on this by putting in lots of mechanisms in place.
(Reviews / Free profession photography / Payment holds to owners until arrival / Social connections / Phone verification / CC Linking )
All of these help create a strong recognition of trust/security (that isn't just an illusion like many other sites).
I'm just not quite sure what additional benefit this offers. (I'll assume it's to protect the home owners more, by requiring guests who place a booking to confirm their identity)
AirBnb don't store any of the verification documents either. They are all stored by Jumio, who also check them against goverment documents to confirm a match. [2]