According to http://www.socialsecurity.gov/employer/randomization.html the Social Security Administration has been centrally issuing all new SSNs since the 1970's, and has been basing the prefix on the postal code from which the application was mailed. In June, they plan to ditch the semantic groupings and start assigning numbers arbitrarily from the pool of unused numbers, so it will lose significance then.
Also, you can apparently request a new SSN if you object to the digits contained in yours: http://ssa-custhelp.ssa.gov/app/answers/detail/a_id/292