MIT is one of many schools that offers free online courses. They are launching MITx, an e-learning program that will offer certificates to students who have mastered the course material. It will not be an MIT certificate, but rather one from a new organization. There will be a modest sum to receive the certificate.
This is just another way schools are reaching out and trying to get more students to take their online courses. Usually, there is no certificate or acknowledgement that you took the course.
Read more here: http://www.openculture.com/2011/12/mit_to_offer_certificates.html