You wouldn't have to keep the print out of the number by your front door, you could take it to work with you if you'd prefer or even just memorise it or give it to the person you want to take delivery for you.
The point is, if a security number that only the customer (and people they trust), vendor and delivery company's handheld know but is never known by the staff of the delivery company it would make it virtually impossible for delivery men to sign packages off themselves and claim they delivered it.
Clearly a signature is no where near secure enough, especially seeing as neither the vendor nor delivery company have an example to compare it against on delivery to authenticate the person.
C'mon, Duncan, are you in?
No hotlinking!