Are they serious? What if you're already an iphone customer? Do you still need to be ready with your freakin social security number to transfer the existing account to the slightly newer iphone???
Who in this country, over the age of 18, does not know their SS#? How difficult is it "to be ready" with it?
If you're that concerned, just don't volunteer it unless the Apple or AT&T rep says there's no way to complete the procedure without it.