Are we really surprised about this? o.O We've always known that SSNs are allocated by states:
http://www.ssa.gov/history/ssn/geocard.html - hell, the government tells us what the next two numbers are likely to be.
And if you know the order that the last four digits are assigned, then knowledge of their birthday and birth time, along with the knowledge of the number of surrounding birth times could give you a good guess as to what someone's SSN might be. :P